A method of making a filament wound pressure vessel. A thermoplastic liner is filament wound with a commingled roving containing thermoplastic and reinforcing filaments. The wound tank is partially filled with a cooling liquid and the wound tank is heated to a temperature which is sufficient to vaporize a portion of the liquid to pressurize the liner at a preselected set pressure and sufficient to fuse the thermoplastic matrix, but insufficient to significantly soften the liner.