Waveguide Mach-Zehnder interferometers (MZI) have been successfully used for a number of sensing applications due to their high sensitivity. As the MZI-sensors have a built-in reference, they are ideally not dependent on temperature variations. However, due to their high sensitivity, a temperature difference or an imbalance between the two arms, can give an unwanted temperature dependence of the output signal. Here, we present an experimental study of the temperature sensitivity of waveguide MZI. Three designs, balanced MZI, unbalanced MZI and tapered MZI are studied. In order to investigate the temperature sensitivity of the interferometer, we measure the phase shift at the output of the interferometer as function of temperature. It is found that the balanced MZI is very stable with temperature. For an unbalanced MZI having a 1 mm length difference between the two arms, a small phase shift is observed. The phase shift was less than one period for a 10°C change. The phase shift can be precisely determined for the tapered MZI. Initial measurements of the temperature sensitivity for a balanced, tapered MZI, gave output variations of some 150° for the phase, for a temperature change of 10°C. This corresponds to a temperature difference of 2.6 mK within the oil covering the two arms and demonstrates how sensitive the device is to temperature differences between the sensing and the reference arms.