OH lines in the FIR spectra of the OH megamaser galaxies IRAS 20100-4156 and 32w35

Using the SWS and LWS instruments on board of the ISO spacecraft, we search ed for OH-lines in the FIR spectra of the two OH megamaser galaxies IRAS 20 100-4156 and 3 Zw 35. The lines at 34 mu and 53 mu were detected in absorpt ion with certainty in the spectrum of IRAS 20100-4156 and probably in the s pectrum of 3 Zw 35. From the measured equivalent widths we derived column d ensitites of N-OH approximate to (2.2 +/- 0.4) . 10(17) cm(-2) and N-OH app roximate to (2.0 +/- 0.6) . 10(17) cm(-2), respectively. The 119 mu line wa s tentatively detected in the spectrum of IRAS 20100-4156, but definitely i s much weaker than would be expected from the strengths of the lines at 34 mu and 53 mu. In the spectrum of 3 Zw 35 the 119 mu line was not found. We take this as an indication that at 119 mu the continuum source is considera bly larger than at 34 mu and 53 mu and is covered only partly by the absorb ing OH cloud. - The finding that the OH column density is similar in both o bjects despite substantial differences in their IR and maser luminosities, is taken as support of the hypothesis that OH megamasers are radiatively pu mped by the 34 mu and/or the 53 mu lines.