We have introduced defects into clean samples of the organic superconductor
κ-(BEDT-TTF)2Cu(SCN)2 in order to determine their effect on the
temperature dependence of the conductivity and the critical temperature Tc. We find a violation of Matthiessen's rule that can be explained by a model
of the conductivity involving a defect-assisted interlayer channel which acts
in parallel with the band-like conductivity. We observe an unusual dependence
of Tc on residual resistivity which is not consistent with the
generalised Abrikosov-Gor'kov theory for an order parameter with a single
component, providing an important constraint on models of the superconductivity
in this material