Although the formation and reactions of gaseous nitrous acid (HONO) in the atmosphere are of great interest, it is difficult to accurately measure HONO both in the atmosphere and in laboratory systems. We report a new technique for quantifying gaseous HOMO in laboratory systems. The method utilizes the reaction of gas phase HOMO with an excess of HCl gas to produce nitrosyl chloride (ClNO), which is readily quantified using FTIR, HONO was formed by flowing N-2 over the surface of an aqueous HCl solution and through a bed of NaNO2, then directly into a 561 L chamber. An excess of gaseous HCl was added to the chamber to initiate the reaction in N-2 at room temperature and 1 atm total pressure. The loss of HONO was followed by DOAS and FTIR and the formation of ClNO was measured by FTIR. While direct measurement of HONO by FTIR is Limited by uncertainties in the available infrared absorption cross sections, calibration for ClNO is readily carried out since ClNO can be synthesized with high purity. The stoichiometry for ClNO formed to HONO reacted was determined to be 0.9 +/- 0.2 (1 sigma). The concentration-time profiles for both HONO and ClNO were fitted with a kinetics model which gave a rate constant for the reaction HONO + HCl (k1)--> ClNO + H2O of k(1) less than or equal to (1.9 +/- 1.3) x 10(-19) cm(3) molecule(-1) s(-1) (2 sigma) at 297 K. This should be taken as the upper limit for the gas phase reaction since some contribution from heterogeneous reaction at the chamber walls cannot be conclusively ruled out.