Abstract In this work we investigated the thermodynamics and kinetics of structural relaxation in a-Si:H films using differential scanning calorimetry (DSC). The relation between structural relaxation and light-induced metastability (Staebler–Wronski effect, SWE) has also been investigated. The a-Si:H films were prepared by RF (13.56 MHz) glow discharge decomposition of the mixtures (10% SiH 4+90% H 2) and (5% SiH 4+95% He), and by 55 kHz plasma-enhanced chemical-vapor deposition (PECVD) of pure silane for high deposition rates. Detailed analysis of the peaks on DSC curves shows that the low-temperature effect (LTEP) is caused by the relaxation of weak Si bonds and that the high-temperature effect is caused by hydrogen effusion with subsequent relaxation of the microstructure. A method for the calculation of kinetic parameters of structural relaxation using DSC data is proposed. We found from the joint investigation of DSC and SWE that both the light-induced defect generation and the LTEP on DSC curves have a common nature and are described by the same reaction between weak Si–Si bonds and dangling bonds. We show that the rates of this reaction in both the forward and the reverse directions are controlled by hydrogen microstructure.