3-Aryl- and 3-alkyl-4-phthalazone-1-carboxylic acids are formed by the reaction of the corresponding mono-substituted hydrazines and phthalic acid, but higher yields of 3-alkyl-4-phthalazone-1-carboxylic acids are obtained by the alkylation of 4(3H)-phthalazone-1-carboxylic acid or its esters. The acids obtained have been characterized by their UV spectra and have been converted into methyl esters and amides (via the chlorides).