An analysis of deep optical echelle spectra towards six stars in the Cyg OB2 (VI Cygni) association is presented. Interstellar absorption lines up to J 00 = 18 in the (2,0) and (3,0) bands of the C2 A 1 u {X 1+ system are detected towards Cyg OB2 No. 12. The large number of rotational lines accurately constrains the gas-kinetic temperature T and the density n to T =3 5 K andn = (600 100) cm 3 . The inferred C2 column density is N(C2 )=( 20 +4 2 )1 0 13 cm 2 . The detection of various lines in the (1,0) and (2,0) band of the CN A 2 u {X 2+ red system suggest a column density of N (CN) =( 8 13) 10 13 cm 2 .C 2 absorption lines are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters are T =5 0 K,n = (600 200) cm 3 , N(C2) = (10 +3:5 1:5) 10