Cavity ringdown spectroscopy of films of oxazine and malachite green dyes coated on thin borosilicate substrates is described. The method involves insertion of the substrate into the cavity at Brewster's angle to reduce reflection losses. Measured absorption features of the adsorbed dyes in the visible region 580 to 700 nm show the presence of dye aggregates. The measurement of sub-monolayers of oxazine 1 as low as 0.032 monolayers with an absorption loss per pass of 5.79 Â 10 À5 is demonstrated
