A study was carried out previously on the effect of diatomaceous earth supports coated with OV-1 stationary phase on the response factors for three typical organophosphorus pesticides, phorate, Disyston, and malathion. There was some indication that the response factors were affected by the column temperature, which was unexpected. Accordingly, tests were undertaken to determine how and why column temperature could affect responses as measured with a electron-capture detector. Such information was expected to be of value in establishing optimum column conditions for the detection of minimal amounts of pesticides. Experiments were carried out using various Chromosrb supports coated with silicone OV-1 stationary phase. Repeated runs were also carried out on one particular column in order to establish the effect of prior usage of a column on its performance in the separation of phorate, Disyston and malathion. Similar bell-shaped thermograms were generally obtained, over the temperature range 140–230°C, regardless of the coated support tested or whether or not the column had previously been used. However, the temperature corresponding to maximum response varied with the support. An initial increase observed in the response factors with increase in column temperature, followed by a decrease in response beyond some critical temperature, was explained by taking into account the following factors: (i) adsorption of the pesticides by the support and/or stationary phase; (ii) bleeding of the stationary phase and its effect on the sensitivity of the electron-capture detector; and (iii) priming by the pesticides of the exposed active sites on the supports. Tailing factors for the recorded peaks were also calculated, and used to confirm the conclusions. It was established that the column temperature to be used to optimize the detection limits for organophosphorus pesticide analyses depended on the nature of the support material. Failure to optimize the column temperature could reduce the sensitivity limits by a factor of, in some cases, as much as five.