Human serum paraoxonase (PON1) exists in 2 major polymorphic forms: Q (glutamine) or R (arginine) at codon 192. The PON1₁₉₂ activity polymorphism is substrate dependent. The PON1_Q192 isoform has a higher rate of in vitro hydrolysis of diazoxon, sarin, and soman, whereas the PON1_R192 isoform has higher activity for the hydrolysis of paraoxon and chlorpyrifos oxon. Both isoforms hydrolyze phenyl acetate at approximately the same rate. The present study described and evaluated a kinetic method of arylesterase activity determination with a modified fixed incubation method that used the oxidative coupling of phenol with 4-aminoantipyrine of phenyl acetate as the substrate. Our improved method shows that arylesterase activity is lower with the PON1_R192 isoform than with the PON1_Q192 isoform. The average activities of serum of individuals of a specific PON1_Q192 genotype showed higher arylesterase and lower paraoxonase activity than the PON1_R192 genotype. The ratio of paraoxonase/arylesterase activity showed a clear separation of all three PON1₁₉₂ genotypes with no overlap between the groups (QQ: < 5.0, QR: 5.0−11.0, RR: > 11.0). PCR has suggested that the PON1₁₉₂ phenotypes correspond to the PON1₁₉₂ genotypes. Therefore, when conducting epidemiological or mechanistic studies that examine the role of PON1 in organophosphorus or lipid metabolism, this ratio is more useful and informative than a PCR-based genotype alone.