The thermonuclear burn-up in deuterium - tritium (DT) mixtures with various D: T ratios is considered. It is shown that the use of small amounts of tritium simplifies the thermonuclear burn-up in the dense deuterium significantly. The general dependence of the critical burn-up parameter upon the D: T ratio, temperature T and initial density is studied. In particular, we found that when the D: T ratio decreases the critical burn-up parameter diminishes very quickly (at fixed T and ), i.e. the thermonuclear burn-up simplifies when the relative tritium concentration in the DT-mixture grows. The thermonuclear burn-up in the highly compressed deuterides and DT-hydrides of light elements is discussed briefly in the appendix.