People who adhere to the custom of not eating kitniyot may keep them in the house during Pesaĥ and derive benefit from them, for example, by lighting a lamp with kitniyot oil (Rema 453:1).
One who does not eat kitniyot on Pesaĥ may cook them for somebody who does, but it is recommended that he use some sort of reminder that he is not cooking for himself (Kaf Ha-ĥayim 453:17; Sidur Pesaĥ Ke-hilkhato 16:8). It is likewise permissible for a storeowner to sell kitniyot during Pesaĥ; however, if there could be wheat grains among the kitniyot that make up more than one sixtieth of the mixture, the storeowner may not sell the kitniyot, because this could cause customers to transgress the ĥametz prohibition. It would be better to sell such kitniyot along with the ĥametz.
If kitniyot fall into a cooked food, they should be removed, and whatever cannot be removed is batel in the majority of the dish. If, however, such a large amount of kitniyot falls in that they become the majority, the dish is considered a kitniyot dish and its consumption is forbidden (Rema 453:1; MB 8-9 ad loc.).
It is permissible for one who does not eat kitniyot on Pesaĥ to eat from and cook with utensils that belong to one who does eat kitniyot, provided that 24 hours have elapsed since kitniyot were last cooked in them. If food was cooked in such a utensil before 24 hours elapsed, the food remains kosher.
. Kitniyot in a mixture are batel in a simple majority (batel be-rov). Although the implication of Terumat Ha-deshen is that it is only batel be-shishim, the Aĥaronim rule that it is batel be-rov (SAH 453:5; Ĥayei Adam 127:1; Ĥavot Ya’ir §6; Eliya Rabba §4; and Kaf Ha-ĥayim 453:25, which also adds more sources). All of this only applies when one found that kitniyot had been mixed with permissible food, but it is forbidden to mix kitniyot with permissible food intentionally, contrary to the opinion of the Pri Ĥadash that one may intentionally add kitniyot to a mixture as long as they do not become the majority of the mixture. If one who does not eat kitniyot visits one who does eat kitniyot and there is nothing else for the guest to eat, he may, if absolutely necessary, eat the non-kitniyot food from a mixture containing kitniyot. For example, he may eat potatoes and zucchini from a dish containing kitniyot, even though the food he is eating absorbed some flavor from the kitniyot.
Even those who abstain from kitniyot are permitted to eat food cooked in a pot in which kitniyot had been cooked within the prior 24 hours, since the taste of the kitniyot is a very small minority. If 24 hours elapsed since the cooking of the kitniyot, one may cook food in the pot even le-khatĥila, since the taste of the kitniyot in the pot has already turned foul, and kitniyot are technically kosher for Pesaĥ anyway (Pri Ĥadash 496:24 and Responsa Maharalbaĥ §121). If one is in doubt whether or not 24 hours passed since the cooking of the kitniyot, he may still use the pot le-khatĥila because of the principle that most pots have not been used in the past 24 hours. See also Kaf Ha-ĥayim 453:27, which lists additional sources.
Some maintain that the prohibition of kitniyot begins at the onset of Pesaĥ (Sidur Pesaĥ Ke-hilkhato 16:10 n. 42), and in time of pressing need one can rely on this opinion. In practice, however, the mainstream opinion is that the custom of kitniyot corresponds to the prohibition of ĥametz. Thus, kitniyot are forbidden from the time that ĥametz is prohibited (Ĥok Yaakov 471:2; Responsa Maharsham 1:183; Shevet Ha-Levi 3, 31).