The relative stability of various sizes of H- and F-terminated c-BN clusters (in relation to h-BN) has been investigated theoretically using the ab initio molecular orbital method at the MP2 order of theory. For comparison, a corresponding investigation has also been performed for H-terminated diamond and graphite clusters. The cubic phase of C (diamond) was for all cluster-sizes investigated found to be more stable than c-BN. Both diamond and c-BN gas phase nuclei were found to be more stabilised for cluster-sizes larger than 6 C (or 3 B+3 N) atoms. This stabilisation energy tends to decrease drastically by using F (instead of H) as terminating species.