The complete series of composites, 25, 50, 75 and 100 wt/wt, of C20 and AG20 matrix with the same reinforcer NSi was measured. The data comparison indicates that the residual mass amount even at 550 °C shows a trend from 20% up to 55% when 75% of reinforcer was added, but it differs significantly when the higher amount of filler is contained, 100%, meaning the 50% of composite total mass; when C20 is the matrix it delivers just 41% of residual mass while in AG20 matrix it is 62 wt %. Such a result suggests that the matrix treated by solvent exchange method is better stabilized by the reinforcer retaining more mass than the matrix that is just untreated.