Abstract: The results of the research on the CIMS conditions by using twelve dialkyl methylp hosphonate compounds are reported in the present paper.Methane,methanoland isobutane were used as the reactant gases to find out the optimum pressure andtemperature, the curves of the relation between the amount of sample and theintensity of the ion(M+1)~+ and the ion(2M+1)~+ were obtained. The sensitivitesof the three reactant gases were compared. A theory basis for selection of reactantgases is proposed here and the law of fragmentation of dialkyl methylphosp-honate compounds is also summarized.