Mental rotation tests have been extensively studied regarding item characteristics that affect difficulty, e.g., angular disparity, item dimensionality, and object complexity. In the present study, using the Vandenberg and Kuse (1978) paradigm, we applied a psychometric approach to examine whether complex skewed-axis rotation requires incremental processes that can be distinguished from simple cardinal-axis rotation. Participants (N = 372) completed a battery of cognitive tests, including a mental-rotation test requiring mental rotation of Shepard and Metzler type figures around a single cardinal axis or around two cardinal axes (resulting in a skewed-axis rotation). When comparing a nested-factor measurement model to a one-factor model, results showed that complex skewed-axis rotation is not identifiable as a nested specific factor. This suggests that the processes resulting in individual differences in mental rotation are either the same in both item types, or at least substantially correlated. Including spatial visualization tests and reasoning tests in a prediction model suggested that participants used spatial strategies over and above reasoning to solve the mental rotation items. These results generalize the findings of Just and Carpenter (1985) on simple cardinal-axis and complex skewed-axis rotation of cubes to more complex objects that allow more flexible mental rotations. It can be concluded that mental rotation represents a unitary ability. From an individual-differences perspective, this ability can be assessed equally with simple cardinal-axis and complex skewed-axis rotation items.