The kinetics of the reversible reaction HONO+NH3 reversible H3N-HONO (1) was studied by monitoring trans-HONO relaxation kinetics. The rate of approach towards equilibrium was studied as a function of the ammonia concentration to obtain values of the rate constants for the forward and reverse reactions as well as the equilibrium constant. At 298 K we have determined the following values for the forward rate constant and the equilibrium constant, k(if)=(2.2+/-0.2) x 10(3) M-1 s-1 and K(c)=(1.5+/-0.2) x 10(5) M-1. The equilibrium constant was determined at temperatures in the range of 287-319 K, and from the observed temperature dependence expressed in terms of the van 't Hoff equation we have derived the following values of DELTAH298(0)=-11.8+/-0.8 kcal/mol and DELTAS298(0)=-26.7+/-1.0 cal/mol K. Theoretical investigations of the structure and thermochemistry of the H3N-HONO hydrogen bonded complex were carried out on the electron correlation level and 6-311 + G(2df, 2pd) basis set. The calculated binding energy is - 10.1 kcal/mol while DELTAH298(0)=-8.1 kcal/mol and DELTAS298(0)=-26.70 cal/mol K.