Newly developed re¯ecting neutron collimators promise increased detector count rates in a ®eld where most work is intensity-limited. The effects of such elements on instrument resolution are complicated and appear to be even more subtle than previously imagined. Numerical tests using the McSTAS ray-tracing program (http://neutron.risoe.dk/mcstas) are reported which support and extend the recent analysis of these effects. If re¯ecting collimators are used in all instrument positions they give unusual scan pro®les. Using re¯ecting collimators only before the monochromator and immediately before the detector has no deleterious resolution effects and increases count rates by a factor of more than two on powder diffractometers.