Microalgae are often considered as a promising alternative source of vegetable oils. These oils can be used for food and biofuel applications. Productivities that are projected for large-scale microalgal oil production are, however, often poorly supported by scientific evidence and based on too optimistic assumptions. To facilitate the inclusion of the microalgal physiology in these projections, existing knowledge and novel scientific insights were condensed into a mechanistic model that describes photosynthesis and carbon partitioning during nitrogen starvation. The model is validated using experimental data from both wild-type and a starchless mutant of Scenedesmus obliquus. The model is subsequently used to quantify how reactor design, process design, and strain improvement can improve the oil productivity from 2.1 to up to 10.9 g m(-2) day(-1). These projected productivities are used to reflect on commonly assumed oil productivities and it is concluded that the microalgal oil productivity is often overestimated several folds. (C) 2015 Elsevier Ltd. All rights reserved.