The effects of γ-irradiation and moisture absorption on the mechanical properties of carbon fibres–epoxy resin composites were studied. The properties dominated by the matrix and fibre–matrix interface (interlaminar and in-plane shear strength) were measured at room temperature using standard tests. These tests were carried out before and after exposures to gamma irradiation and before and after immersion in water at 80°C during 21 days. The dosage of gamma irradiation was up to 11.7 MGy. The micrographs of surfaces fractured in performed tests were observed on a scanning electron microscope. They were analyzed with consulting the stated effects on mechanical properties and the measured values of the glass transition temperature of tested coupons before and after irradiation and immersion in water. The obtained results show that moisture and irradiation, if they act one after the other, have a significant influence on the degradation of matrix-dominated mechanical properties of the tested carbon–epoxy composite.