We developed a method to assess the substitutability of two reinforcers by using the divergence of the cross point of two demand functions. Two kinds of water were used as reinforcers, namely distilled water and quinine water. We tested 16 rats, Rattus norvegicus, from two strains in a closed economy. A single demand function for each kind of water was established. Then, two reinforcers were presented on concurrent fixed-ratio schedules. Finally, a control condition with distilled water for both responses was run. Demand functions were generated on scales with fixed-ratio values on the X axis and number of reinforcers obtained on the Y axis. The cross point of the functions differed significantly between the two strains of rats in both conditions within each condition. Furthermore, there was a significant difference within strains between the conditions. On evaluating the single demand functions, we found a significant difference between the slopes of the two demand functions, but no strain differences in demand. In addition, the results revealed a disagreement between demand assessed by using the slope of the single demand function and the results using the double demand function, with the results of the double demand function being in accordance with simple choice behaviour of the rats in their home environment. Using the cross point of two demand functions provides a measure of substitutability and, furthermore, the method appears to be a more sensitive measure of animal priorities than single demand functions.