The enzymatically catalyzed oxidative methylation of As yields methylated arsenicals that contain pentavalent As (AsV). Because trivalent As (AsIII) is the favored substrate for this methyltransferase, methylated arsenicals containing AsV are reduced to trivalency in cells. Methylated arsenicals that contain AsIII are extremely potent inhibitors of NADPH-dependent flavoprotein oxidoreductases and potent cytotoxins in many cell types. Therefore, the formation of methylated arsenicals that contain AsIII may be properly regarded as an activation step, rather than a means of detoxification. Recognition of the role of methylated arsenicals that contain AsIII in the toxicity and metabolism of As emphasizes the need for analytical methods to detect and quantify these species in biological samples. Hence, a method was developed to exploit pH-dependent differences in the generation of arsines from inorganic and methylated arsenicals that contain either AsV or AsIII. Reduction with borohydride at pH 6 generated arsines from inorganic AsIII, methyl AsIII, and dimethyl AsIII, but not from inorganic AsV, methyl AsV, and dimethyl AsV. Reduction with borohydride at pH 2 or lower generated arsines from arsenicals that contained either AsV or AsIII. Arsines are trapped in a liquid nitrogen-cooled gas chromatographic trap, which is subsequently warmed to allow separation of the hydrides by their boiling points. Atomic absorption spectrophotometry is used to detect and quantify the arsines. The detection limits (ng As ml−1) for inorganic AsIII, methyl AsIII, and dimethyl AsIII are 1.1, 1.2, and 6.5, respectively. This method has been applied to the analysis of arsenicals in water, human urine, and cultured cells. Both methyl AsIII and dimethyl AsIII are detected in urine samples from individuals who chronically consumed inorganic As-contaminated water and in human cells exposed in vitro to inorganic AsIII. The reliable quantitation of inorganic and methylated arsenicals that contain AsIII in biological samples will aid the study of the toxicity of these species and may provide a new biomarker of the effects of chronic exposure to As.