In this paper, we present the first high-resolution spectra and analysis of the nu(4) fundamental bands of fully O-18-substituted nitrous acid, trans- and cis-(HONO)-O-18-O-18. These bands are not perturbed by neighboring vibrational levels and were used to determine for the first time accurate rotational and centrifugal distortion constants of the ground and nu(4) = 1 states of trans- and cis-(HONO)-O-18-O-18. The ground-state rotational constants were then used. together with the rotational constants of other HONO isotopic species and with rotation-vibration parameters from ab initio calculations, to determine accurate semi-experimental equilibrium structures of trans- and cis-HONO. Our study confirms the results of a recent work by Demaison et al. (J. Phys. Chem. A 2006, 110, 13609-13617) concerning the structure of trans-HONO, whereas the new structure of cis-HONO obtained in this paper is a significant improvement compared with the previous work of Cox et al. (J. Mol. Struct. 1994, 320, 91-106). The recommended parameters for the equilibrium structure of cis-HONO are r(e)(O=N) 1.1816(10) angstrom, r(e)(N-O) = 1,3887(10) angstrom, r(e)(O-H) = 0.9744(7) angstrom. angle(e)(ONO) = 113.18(1)degrees, and angle(e)(HON) 104.67(4)degrees.