We investigated the bonding-related gases trapped inside the cavities of micromachined silicon test structures that had been sealed by silicon direct bonding or anodic bonding under vacuum conditions. The gas content inside the cavities was analyzed by quadruple mass spectroscopy The magnitude of the residual gas pressure inside the cavities for different cavity layouts and for various bonding processes was monitored. In cavities bonded by low-temperature silicon direct bonding the residual gases are reaction products originating from the mating silicon surfaces during annealing. Inside the cavities mainly H-2, H2O and N-2 are found. The total gas pressure is primarily determined by the H-2 component. Cavities sealed by anodic bonding mainly contain O-2, which originates from mobile oxygen ions inside the bonding glass. The residual gas pressure inside anodically bonded cavities depends neither on the applied bonding voltage nor on the bonding area surrounding each cavity.