A number of methods for making improvements to optical solar reflectors for passive thermal control of spacecraft have been discussed in previous papers (1, 2). These methods have been evaluated practically in terms of their likely increase in efficiency against the degree of complexity require to incorporate the improvement itself. Those which appear to be advantageous have then undergone full development, qualification and productionisation. The first of these, the low absorptance CMX OSR, with an ∝/ε ratio has been reduced to half its original value, has recently completed full qualification by the Royal Aerospace Establishment in the UK. The work forms part of a programme of continual upgrading of OSRs with regard to optical, thermo-optical, mechanical and electrical properties but this paper will concentrate on considering the most important property of an OSR; its thermal efficiency.