•Efficiency of anaerobic digestion is hampered due to microalgae cell wall.•Organic matter solubilisation and improved biodegradability was achieved by all thermal pretreatments.•Thermal pretreatments enhanced hydrolysis rate constant and thus faster biogas production.•Carbohydrates solubilised provided a fairly close estimation for methane production.•Best case scenario was achieved by subjecting biomass to 160°C which resulted in 64% methane yield enhancement. This study investigates the effect of high pressure thermal hydrolysis on organic matter solubilisation and biogas production from Chlorella vulgaris biomass. Microalgae biomass was subjected to three temperatures, namely 140, 160, and 180°C and two heating times (10 and 20min). Results showed that carbohydrates release prevailed over proteins. Carbohydrates were solubilised concomitantly with increasing temperatures. According to the infrared spectra and monomeric sugars determined in the pretreated medium, temperatures applied clearly affected the solubilisation of structural carbohydrates of the microalgae cell wall. Likewise, thermal pretreatment provided enhanced methane production with regard to the raw algal biomass. Enhanced hydrolysis rate constant supported faster biogas production. Regardless the heating time employed, increasing temperatures depicted increasing methane production. Even thought, organic matter solubilisation was greater at 180°C, the anaerobic biodegradability did not show the same trend. This fact was ascribed to the formation of reaction products that hampered methane production. Best case scenario was achieved by subjecting biomass to 160°C which resulted in 64% methane yield enhancement.