Simultaneous differential thermal analysis and thermogravimetry (DTA-TG) was applied to quality control of nylon 6,6 composites reinforced with mixed glass and carbon fibres. A methodology is described for the evaluation of 13 parameters of virgin and waste composite materials. Three significant parameters were identified for quality control of the composites using statistical methods. The application of the testing method to prepared test composites showed a good agreement between results and performance of this composite using impact testing. Simultaneous DTA-TG is a fast, small sample-consuming tool for quality control testing on the title composites and is potentially appropriate for other composites reinforced with mixed fibres.