The valence band offset (Delta E-v) between strained Si1-xGex and Si can be determined by temperature-dependent current-voltage (I-V-T) measurements on Si/Si1-xGex/Si quantum wells (QW) using two kinds of contacts. Firstly, we used samples with ohmic contacts and with a simple theory of thermionic emission we estimated Delta E-v = 147 +/- 10 meV for x = 0.17 and Delta E-v = 190 +/- 10 meV for x = 0.22. Secondly, we investigated samples with Schottky contacts. The forward I-V-T characteristics show a normal Schottky diode behavior up to 0.5 V, Then the slope decreases strongly at higher voltages. This behavior was attributed to the effect of an additional barrier due to the band offset between Si and Si1-xGex at the QW. From these measurements we estimated Delta E-v = 105 +/- 20 meV for x = 0.17 and Delta E-v = 190 +/- 20 meV for x = 0.22. It is shown that the Delta E-v evaluated on samples with ohmic contacts an more reliable.