Maximal α-leakage is a tunable measure of information leakage based on the accuracy of guessing an arbitrary function of private data based on public data. The parameter α determines the loss function used to measure the accuracy of a belief, ranging from log-loss at α = 1 to the probability of error at α = ∞. To study the effect of side information on this measure, we introduce and define conditional maximal α-leakage. We show that, for a chosen mapping (channel) from the actual (viewed as private) data to the released (public) data and some side information, the conditional maximal α-leakage is the supremum (over all side information) of the conditional Arimoto channel capacity where the conditioning is on the side information. We prove that if the side information is conditionally independent of the public data given the private data, the side information cannot increase the information leakage.