Several new 4‐amino‐5‐arylsulfonylpyrimidines were prepared via the reaction of various α‐(ethoxymethylene)arylsulfonylacetonitriles with guanidine or variously substituted amidines (Table II). 2,4‐Diamino‐5‐(p‐chlorophenylsulfonyl)pyrimidine (IIIg), a typical example, was prepared from the reaction of 2‐(p‐chlorophenylsulfonyl)‐3‐ethoxyacrylonitrile (IId) with guanidine in refluxing ethanol containing sodium ethoxide. With proper substitution of the ethoxymethylene intermediate, the method was found suitable for the preparation of other compounds having either a hydroxy or methyl group at the 4‐position of the 5‐arylsulfonylpyrimidine. The fluoro group in 2,4‐diamino‐5‐(p‐fluorophenylsulfonyl)pyrimidine (IIIx) was successfully replaced by nucleophilic reagents such as sodium ethoxide, N‐methylpiperazine and N,N‐diethylethylene‐diamine. Attempts at direct displacement of fluorine by ammonia at 190° were unsuccessful.