Spectra of hypochlorous acid, HOCl, have been measured in the 30-210 cm -1 interval with a practical resolution of 0.002 cm -1. The spectroscopic analysis led to the assignment of 868 ground state rotational transitions (up to J = 54 and Ka = 6) for HO 35Cl, and 775 transitions (up to J = 49 and Ka = 6) for HO 37Cl. Moreover, 327 pure rotational transitions (up to J = 5) within the v3 = 1 vibrational state of HO 35Cl were also assigned. Improved sets of molecular constants were obtained for the ground states of both isotopic species from the simultaneous analysis of our far-infrared measurements and the microwave and millimeter-wave data presently available. On the other hand, the parameters derived for the v3 = 1 state are very similar to those reported in the literature.