The formation of volatile products during the reaction of As(III: ), As(V: ), MeAsO(OH)(2), and Me(2)AsO(OH) with aqueous NaBH(4) has been investigated, and the formation of arsanes, diarsanes, and triarsanes has been detected. The presence of triarsanes is reported here for the first time. Diarsanes and triarsanes are likely formed in condensation cascade reactions, whereas trimethylarsane arises via the transfer of a methyl group. The formation of volatile by-products is considerably reduced by increasing the acidity of the medium and the concentration of NaBH(4) or by the addition of thiols, such as cysteine. A reaction scheme is proposed which reconciles the evidence reported herein and elsewhere in the literature that is valid for both analytical (trace analysis) and non-analytical reaction conditions.