1-Alkylpyridinium bromides 1 having activated N-methylene group react with phosphorus trichloride to give N-(dichlorophosphinomethylene)pyridinium ylides 2. The site of the reaction in 1,2-dialkylpyridinium halides 3 under these conditions is determined by the relative activation of 1- and 2-methylene groups; in the absence of sufficient activation of N-methylene group, reaction occurs at the 2-methylene group to give dichlorophosphinylated anhydrobases 5 and 11. 1,4-Dialkylpyridinium bromide 13 behaves analogously to give the corresponding dichlorophosphinylated anhydrobase 14.