Using the IRAM 30 m telescope we observed molecular absorption lines from CCH and c-C 3 H 2 produced by diffuse and translucent clouds along the lines of sight towards massive star forming regions. The same sources are surveyed with Herschel/HIFI as part of the PRISMAS guaranteed time program, for molecular absorption lines due to hydrides and carbon clusters. The background sources are massive star-forming regions (G34.3 + 0.1, G10.62 − 0.39, W51, W49N) and SgrA * . The line profiles of the

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... and c-C 3 H 2 are strikingly similar for all lines of sight, showing that the ratio of the opacities of the probed transitions, (J Ka,Kc = 2 1,2 −1 0,1 ) for c-C 3 H 2 and (J = 1−0, F = 5/2−3/2) for CCH, is nearly constant along all lines of sight, at τ CCH ∼ 1.8 × τ c-C 3 H 2 . As a consequence, the ratio of the column densities of CCH and c-C 3 H 2 is nearly constant and similar to the value derived earlier for diffuse clouds detected along lines of sight towards extragalactic continuum sources, N(CCH) = (28 ± 1.4)N(c-C 3 H 2 ) (Lucas & Liszt 2000 , A&A, 358, 1069. PDR models are able to reproduce the observed CCH column densities for the range of physical conditions appropriate for the absorbing matter (n = 100−3000 cm −3 ; A V = 1−5 mag) but can neither fit the observed c-C 3 H 2 column densities nor the tight correlation between CCH and c-C 3 H 2 .