Dynamic stall on an oscillating airfoil was investigated by a combination of surface pressure measurements and time-resolved particle image velocimetry. Following up on previous work on the onset of dynamic stall (Mulleners and Raffel in Exp Fluids 52(3):779-793, 2012), we combined time-resolved imaging with an extensive coherent structure analysis to study various aspects of stall development. The formation of the primary dynamic stall vortex was identified as the growth of a recirculation region and the ensuing instability of the associated shear layer. The stall development can be subdivided into two stages of primary and secondary instability with the latter being the effective vortex formation stage. The characteristic time scales associated with the primary instability stage revealed an overall decrease in dynamic stall delay with increasing effective unsteadiness of the pitching airfoil. The vortex formation stage was found to be largely unaffected by variations of the airfoil's dynamics. © 2013 Springer-Verlag Berlin Heidelberg.