We experimentally demonstrate how the thermal crosstalk between active and passive components limits the performance of integrated Mach–Zehnder (MZ) modulators operating in the radiofrequency regime. To evaluate the role of the distance between active and passive components, the MZs are placed at different distances with respect to a semiconductor optical amplifier that represents the heat source. The thermal crosstalk is quantified by measuring the effects on the electro–optical response of MZ modulators considered as the test structure. Both extinction ratio and bit error rate degradation are measured. The proposed investigation allows the introduction of design rules, based on the minimum distance between components to ultimately avoid unwanted thermal effects. According to the result provided by our analysis, we also show how the ER degradation can be recovered.