The toxicity of oil sands process-affected water (OSPW) has regularly been attributed to naphthenic acids, which exist in complex mixtures. If on remediation treatment (e.g., ozonation) or on entering the environment, the mixtures of these acids all behave in the same way, then they can be studied as a whole. If, however, some acids are resistant to change, whilst others are not, or are less resistant, it is important to establish which sub-classes of acids are the most toxic. In the present study we therefore assayed the acute toxicity to larval fish, of a whole acidified OSPW extract and an esterifiable naphthenic acids fraction, de-esterified with alkali: both fractions were toxic (LC50 ∼5-8mgL(-1)). We then fractionated the acids by argentation solid phase extraction of the esters and examined the acute toxicity of two fractions: a de-esterified alicyclic acids fraction, which contained, for example, adamantane and diamantane carboxylic acids, and an aromatic acids fraction. The alicyclic acids were toxic (LC50 13mgL(-1)) but the higher molecular weight aromatic acids fraction was somewhat more toxic, at least on a weight per volume basis (LC50 8mgL(-1); P<0.05) (for comparison, the monoaromatic dehydroabietic acid had a LC50 of ∼1mgL(-1)). These results show how toxic naphthenic acids of OSPW are to these larval fish and that on a weight per volume basis, the aromatic acids are at least as toxic as the 'classical' alicyclic acids. The environmental fates and other toxic effects, if any, of the fractions remain to be established.