This paper compares the conception of substantial change put forth by Peter of Mantua (d. 1399) in his De primo et ultimo instanti with the one developed by Albert of Saxony (ca. 1320–1390). According to Albert, (i) each substantial form, save for the intellective soul, is a spatially-extended entity with actual quantitative parts that are co-located with the parts of matter they inform, and (ii) these quantitative parts are generated and corrupted one after another over an extended interval of time. (...) From (i) and (ii), Albert deduces that (iii) the substantial change of material substances as individuals (save for human beings) is a temporally extended process. This is what I call the ‘piecemeal’ conception of substantial change. I show that Peter of Mantua is significantly indebted to this conception. Indeed, he endorses both (i) and (ii). Moreover, he even extends this approach to the process by which the intellective soul informs matter and is separated from it. Nevertheless, I argue that, thanks to his peculiar doctrine of numerical sameness over time, Peter rejects (iii) and maintains, instead, that the substantial change of any material substance as an individual is an instantaneous event. (shrink)