Current work with the cavity phase shift (CAPS) method concentrates on hydrogen fluoride laser wavelengths between 2.6 and 3.0 um. This method and continued advancements in experimental techniques are described. With use of a cavity dither the accuracy of reflectance measurements has been improved over previous work to ±0.0004 for reflectances around 0.995. Results of measurements on dielectric-coated elements are reported, as are studies on metallic monolayers placed on nontransmitting substrates. The required CAPS configuration in these cases is a three-mirror arrangement permitting angle-of-incidence variation. In this mode the CAPS method becomes a verification technique for optical coatings on particular resonator designs.