Many acid herbicides must be methylated before analysis by gas-liquid chromatography. Several reagents were evaluated for the esterification of substituted acid herbicides. Esterification by the different methods was determined on 2,3,6-trichlorophenylacetic acid [fenac], 2,3,6-trlchlorobenzoic acid [2,3,6-TBA], 4-amino-3,5,6-tri-chloroplcollnlc acid [picloram], 2-methoxy-3,6-dlchlorobenzolc acid [dlcamba], and 3,6-dichlorosalicylic acid, a possible metabolite of dlcamba. The effect of substltuent size and position is discussed in relation to esterification. The best methylating reagent for acid herbicides was diazomethane although it is toxic, explosive, and more costly than the other methods. BF3 -methanol was equally as good when the compound was not sterlcally hindered. Water interfered with the esterlfication efficiency of all reagents. Diazomethane esterified the ammonium salt but not the sodium salt of the acids used.