This paper addresses the problem of mode-mixity for dynamic crack initiation experiments. We develop further our previously introduced approach based on the dynamic path independent H-integral and its experimental implementation on the Compact Compression Specimen (CCS) (Bui et al., 1992, Int. J. Solids Structures, 29(23), 2881–2895). Specifically, a procedure for mixed-mode separation into symmetric and anti-symmetric components of the H-integral is presented and verified numerically. Each component is shown to verify path-independence. The corresponding stress intensity factors are calculated and it is noted that, regardless of the crack length, mode I is the predominant crack opening mode past a short period of quantified mode-mixity. The procedure for mode-separation is applied to two experiments carried out on steel and PMMA (one specimen of each material). In these experiments, it is shown that crack initiation occurred effectively under dominant mode I. Consequently, the complete evolution of the mode I and II stress intensity factors can easily be determined in experimental testing of CCS. Correspondingly, provided we have accurate determination of the time to fracture, the crack initiation mode can be characterized.