We successfully modified a ferric hydroxamate spectrophotometry method for assaying glycolic acid. Comparable to the high-performance liquid chromatography (HPLC)-based method, ferric hydroxamate spectrophotometry can be used to accurately monitor the time course of glycolonitrile bioconversion. Glycolic acid was assayed simply and rapidly at room temperature ($25{sim}35^{circ}C$). Optimum culture conditions were obtained using this method to assay the glycolonitrile-hydrolyzing activity of Rhodococcus sp. CCZU10-1. The preferred carbon and nitrogen sources and ideal inducer were glucose (10 g/L), a composite of peptone (10 g/L) plus yeast extract (5 g/L), and ${varepsilon}$-caprolactam (2 mmol/L), respectively. The optimal growth temperature and initial medium pH for Rhodococcus sp. CCZU10-1 glycolonitrile-hydrolyzing activity were $30^{circ}C$ and pH 7.0. Modified ferric hydroxamate spectrophotometry could potentially be employed to assay other carboxylic acids.