The question whether equilibrium exists between fluids in different parts of an oil or gas-condensate reservoir is often difficult to answer. A new simple model has been developed that can answer this question. The method uses compositional data from two wells or zones in a reservoir and a predictive characterization for the plus fraction. No adjustable parameters are used. Comparing the compositions in the two phases and calculating the fugacity coefficients of each component (using a cubic equation of state), a graphical presentation can be made that clearly shows whether the fluids are in equilibrium or not. The effect of gravity and the reservoir temperature gradient on the compositional gradient in the oil column as well as change in composition due to the production prior to sampling is taken into account. The method has been applied to a low-permeability reservoir where large variations in GOR were observed. It is concluded that the variation in composition and GOR of fluids from different parts of the reservoir could not be accounted for by the effects of gravity, temperature, nor production prior to sampling. The oil originating from different parts of the reservoir was apparently not in equilibrium prior to initiation of production.