The breakdown behaviour of SiPMs (Silicon PhotoMultiplier) with pixel sizes of 15 x 15, 25 x 25, 50 x 50, and 100 x 100 mu m(2), manufactdred by KETEK, has been investigated. From the current-voltage characteristics measured with and without illumination by LED light of 470 nm wavelength, the current-breakdown voltage, V-I, and from linear fits of the voltage dependence of the SiPM gain, measured by recording pulse-area spectra, the gain-breakdown voltage, V-G, have been obtained. The voltage dependence of the Geiger-breakdown probability was determined from the fraction of zero photoelectron events with LED illumination. By comparing the results to a model calculation, the photodetection-breakdown voltage, V-PD, has been determined. Within experimental uncertainties, V-I and V-PD are equal and independent of pixel size. For V-G, a dependence on pixel size is observed. The difference V-I - V-G is about 1 V for the SiPM with 15 mu m pixels, decreases with pixel size and is compatible with zero for the SiPM with 100 mu m pixels. (C) 2016 Elsevier B.V. All rights reserved.