The quality of different foods has been found in previous studies to effect an animal’s performance on fixed-ratio schedules. Foods that are of a higher quality should maintain behaviour to larger fixed-ratio values than foods of a lower quality. The present study examined the performance maintained by increasing fixed-ratio schedules by four different foods (Rolled Oats, Cocoa Puffs™ and flaked barley, All Bran™ and Soy Protein) with six brushtail possum (Trischosurus vulpecula). Overall response rates and running response rates both generally showed a bitonic function, and post-reinforcement pauses showed a modest increase with increases in the fixed-ratio. The equations of two quantitative models; Behavioural economics and mathematical principles of reinforcement (MPR) were fitted to the data to see how different food types affected the two models ability to predict behaviour. This was measured through two parameter estimates, alpha (α) and specific activation (αs). Both demand equations and the MPR equation described the data fairly well. The parameter estimates for specific activation (αs) showed a significant difference in value across the foods, but there was no significant difference across foods for parameter estimates of alpha (α). There was a weak correlation between parameter estimates of α and αs (r =-0.245, p =0.0926) . The four foods used in this study were also tested in a paired-stimulus preference assessment. The foods that were assessed as being of more value in the behavioural economic equations and the MPR model, for some possums were identified as being more preferred, but this was not consistent for all possums.