A pair of 1.5μm semiconductor laser frequency standards have been developed for optical telecommunications use, stabilised to transitions of 12 C 2 H 2 and 13 C 2 H 2 , using cavity-enhanced Doppler-free saturation absorption spectroscopy. The absolute frequencies of 41 lines of the ν 1 +ν 3 band of 12 C 2 H 2 , covering the spectral region 1520–1545nm, have been measured by use of a passive optical frequency comb generator, referenced to 13 C 2 H 2 transitions of known frequency. The mean experimental uncertainties (coverage factor k=1) of the frequency values are 3.0kHz (type A) and 10kHz (type B). Improved values of the band origin ν 0 , rotational constants B′ and B″, and centrifugal distortion coefficients D′, D″, H′, and H″ are presented.