We study the mechanisms of strain-induced InAs quantum dot (QD) formation using theoretical and experimental techniques with focus on the influence of the growth parameters such as temperature and flux. The QDs are grown using solid-source molecular-beam epitaxy on GaAs and AlAs substrates and investigated with in situ electron diffraction, x-ray diffraction techniques, and atomic force microscopy. The experimental data of the critical time up to quantum dot formation and of the QD structural properties are modelled in terms of a kinetic rate-equations-based growth model. We distinguish three main temperature regimes. At low temperatures (T ≤ 420°C), QD formation is assumed to be mainly controlled by kinetic migration of adatoms on the surface. At higher temperatures, the additional process of intermixing of the QDs with substrate material is observed, which crucially modifies the QD formation process. Due to this intermixing, the strain energy inside the dots is significantly reduced and, accordingly, the driving force for QD formation. As a consequence, the critical time for QD formation increases. At T > 520°C for InAs on GaAs and T > 540°C for InAs on AlAs, desorption of In from the QDs becomes important and yields a further delay of QD formation.