In Western tonal music, voice leading (VL) and harmony are two central concepts influencing whether a musical sequence is perceived as well-formed. However, experimental studies have primarily focused on the effect of harmony on the cognitive processing of polyphonic music. The additional effect of VL remains unknown, despite music theory suggesting VL to be tightly connected to harmony. Therefore, the aim of this study was to investigate and compare the effects of both VL and harmony on listener expectations. Using a priming paradigm and a choice reaction time task, participants (N = 34) were asked to indicate whether the final chord in a sequence had a different timbre than the preceding ones (cover task), with the experimental conditions being good and poor VL or harmony, respectively. An analysis with generalised mixed effects models revealed a significant influence of both VL and harmony on reaction times (RTs). Moreover, pairwise comparison showed significantly faster RTs when VL was good as compared to both VL and harmony being poor, which was not the case when only harmony was good. This study thus provides evidence for the additional importance of VL for the processing of Western polyphonic music.