We describe synthesis and characterization of a derivative of aza-m-xylylene, diradical 2, that is persistent in solution at room temperature with the half-life measured in minutes (similar to 80-250 s) and in which the triplet ground state is below the lowest singlet state by >10 kcal mol(-1). The triplet ground states and Delta E-ST of 2 in glassy solvent matrix are determined by a new approach based on statistical analyses of their EPR spectra. Characterization and analysis of the analogous diradical 1 are carried out for comparison. Statistical analyses of their EPR spectra reliably provide improved lower bounds for Delta E-ST (from >0.4 to >0.6 kcal mol(-1)) and are compatible with a wide range of relative contents of diradical vs monoradical, including samples in which the diradical and monoradical are minor and major components, respectively. This demonstrates a new powerful method for the determination of the triplet ground states and Delta E-ST applicable to moderately pure diradicals in matrices.