Sub-optimal choice procedures are commonly used to investigate an animal model of human gambling, and the numerous variables which can affect choice responding (Zentall, 2011). This procedure typically presents two alternatives, one which provides less overall reinforcement than the other. This study aimed to examine whether manipulating terminal link duration on both alternatives would have an effect on possums level of preference towards either alternative, and whether possums would attend to overall reinforcement probability, or discriminative stimuli. Overall, subjects responded optimally at all terminal link durations, regardless of position of the optimal alternative. Response latencies towards the non-preferred alternative were longer than those towards the preferred alternative. Response rates towards both stimuli on the discriminative alternative were similar, indicating that possums were not discriminating between these stimuli. This suggests that terminal link duration does not affect preference when it is altered on both alternatives. The results of this study along with other research in this area suggests that the variables that appear to have the strongest influence on sub-optimal choice are impulsivity and reinforcer deprivation levels, or motivating operations.