A preform resin immersion process (PRIP) for impregnating resin into a preform for the fabrication of composite structures. The resin contains a heatactivated catalyst and a thixotropic agent which maintain the viscosity of the resin at selected low level during the resin immersion process. The dry preform is placed in an impregnation vessel, and the resin is held in a resin storage vessel, with both vessels initially held at vacuum of about 5.0 mm Hg torr to draw off any air or volatiles. Pressure is introduced into the resin storage vessel, causing the resin to flow through a line into the impregnation vessel which is still held at vacuum conditions. When enough resin is in the impregnation vessel to completely immerse the preform, the flow of resin to this tank is stopped. Once the resin has completely permeated the preform, the resin is drawn back into the resin storage vessel, and the saturated preform is raised up from the impregnation vessel so that the excess resin drips off into collection means. By carefully controlling the thixotropic nature and the viscosity of the resin along with the employment of a latent, heat-activated catalyst, the precisely correct amount of resin remains in the preform after the drip collection step is finished. The saturated preform is then conveyed to a conventional tool for final curing at high temperature and pressure.