Single and joint effects of hydrocarbons and a shoreline cleaning agent (SCA) were studied by measuring the inhibition of the larval growth of sea urchin. Different dosage methods of hydrophobic compounds were compared. The results obtained in the evaluation of CytoSol toxicity revealed that the method of variable dilution of water accommodated fraction (WAF) led to the more conservative toxicological approach. Regarding to Libyan oil, the use of DMSO as carrier allowed us the evaluation of its potential toxicity in comparison with the limitations imposed to the use of WAF method. A reparametrised form of the Weibull equation was slightly modified to be useful for dose-response analysis. This was the basis for modelling single sigmoid responses, which were used to simulate biphasic profiles with addition of effects and to describe both the concentration addition (CA) and independent action (IA) hypotheses. In all cases, its descriptive ability was graphically and statistically satisfactory. The IA model was the best option to explain the combined experimental responses obtained. (C) 2010 Elsevier B.V. All rights reserved.