The existence of multiple reflections in TeraHertz optical system causes numerous problems in applications ranging from astronomical to medical instrumentation. We have performed a detailed theoretical study, using waveguide and free-space modal matching, of the high-Q modes that appear on THz optical paths. Highly accurate measurements at 0.48 THz reproduce in detail the complex behaviour that was predicted theoretically. This work opens up the possibility of designing schemes for removing these troublesome effects. Â© 2004 IEEE.