Alteration of the refractive index of benzocyclobutene film by oxygen-ion implantation has been investigated. An increase in the refractive index of 3.8% was observed with an oxygen ion dose of 3 X 10(14) cm(-2). The optical loss of the ion-implanted region was measured using both surface planar waveguides and channel ones. With the latter structure, a 2.3 dB/cm propagation loss was obtained with an oxygen-ion dose of 10(14) cm(-2). The refractive index increase and the optical loss were stable against heat treatments. It was deduced that the refractive-index increase can be attributed to compression of the firn and additionally to oxidation of the film.