Residual organophosphorus pesticides (OPP) in rice decrease when rice is cooked, but the extent of the decrease varies largely with the kind of pesticide used, ranging from 20% for Dimethoate to 93.5% for Ronnel. To determine the mechanism of this decrease, 14 kinds of pesticides were tested in a model experiment designed to determine their thermal decomposition and steam distillation behavior. From the results of the experiment, the test pesticides can be classified into three groups: 1) compounds resistant to both thermal decomposition and steam distillation (e. g., Dimethoate), 2) compounds thermally stable but susceptible to steam distillation (e. g., Fenitrothion=MEP), and 3) compounds thermally unstable and susceptible to steam distillation (e. g., Diazinon). The experimental results accounted well for the behavior of residual. OPPs in rice during the cooking process, and the residual amounts of OPPs after cooking can be estimated from the results.