A quantum mechanical eigenchannel theory of unimolecular dissociation is outlined with specific reference to systems with many closed and few open channels. The resonances (whose lifetimes determine state specific dissociation rates) are shown to decay at rates determined by the fractional open channel weightings in the relevant eigenchannels, while the relative weightings between the open channels determines the fragment internal state distribution. An isolated resonance, random eigenvector version of the theory bears a similarity with established statistical theories. A simplified model application to the in plane dissociation of HCN is reported, for which 60 channels are required for convergence. The decay rates for different resonances are found to be broadly statistical, but the fragment state distributions vary markedly from one resonance to another even when dynamical interactions between many overlapping resonances are taken into account.