Diamondlikea‐C:H films have been deposited by plasma decomposition of C2H2. 50 samples were prepared with a systematic variation of the deposition parameters: the substrate bias voltage was between −100 and −1400 V and the C2H2gas pressure was between 4×10−4and 2.6×10−1mbar. The following properties of the films were measured: the density by Rutherford backscattering, the total concentration of hydrogen by elastic recoil detection, the bonding ratiosp3/sp2by infrared spectroscopy, the internal stress by the bending beam method, and the hardness with a Knoop microhardness tester. It has been shown that the hardness and other mechanical properties cannot be correlated to the average carbon coordination numbermc. This is becausemcis calculated under the assumption of a homogenous single‐phase model, which does not seem to be justified. It is demonstrated that the mechanical properties can be explained by the application of a void model.