The compound p‐chlorophenyl methylcarbamate (PCMC) was shown to inhibit the breakdown of the herbicide isopropyl 3‐chlorophenylcarbamate (chlorpropham) in soil, increasing its half‐life from 7.5 to 20 days. The inhibitory effect of PCMC on the breakdown of chlorpropham persisted for 40 days, with a half‐life of 20 days. The conversion in soil, of PCMC to p‐chlorophenol, a compound which did not affect the persistence of chlorpropham, was demonstrated, and it is suggested that spontaneous chemical hydrolysis may account for this conversion. PCMC and p‐chlorophenol were shown to be of low toxicity to buckwheat (Fagopyrum esculentum Moensch.).