The spreading properties of several hydrocarbons (vegetable,engine, gear and crude oils) on distilled and artificial seawater were determined under laboratory conditions using a noveloptical method. With the aid of Langmuir's equation, the geometricalsignatures of a discrete lens of each hydrocarbon droplet floatingon a water tank served to calculate the entering<i>E</i> (31.30-94.18 mN m^-1) and spreading<i>S</i> (-3.50 to -57.49 mN m^-1) coefficients, and equilibriumthicknesses t_∞ (0.20-1.25 cm). They appeared to be in agreementwith the values derived from direct interfacial tension measurements (Wilhelmyplate and stalagmometer methods). Empirical relations of the normalized lensradius r_L / r_drop and <i>S</i> on the water surface tensionγ_AW were postulated as being of significant value in oil spillassessment studies at sea. The parameters obtained together with the surfaceproperties of a natural surfactant-containing water body represent theprincipal input data required for modelling the spreading of asurface-tension-gradient-driven oil spill at sea.