The reaction of arylbiguanides and N-amidino-O-alkylisoureas with diketene was found to proceed through the acylated intermediates to give pyrimidines and s-triazines. The reaction of arylbiguanides with ethyl acetoacetate was also attempted and the proportion of pyrimidines and s-triazines obtained was found to be almost equall to that in the case of the reaction with diketene. Therefore, this reaction was also elucidated to proceed via the acylated intermediates.