Nitrous acid is one of the simplest molecules that undergoes geometric isomerization. The reaction path properties and rate constants of isomerization have been determined at the MP2 and DFT levels of theory. Good agreement is obtained with experimental data. The calculated isomerization threshold energy is 10.8-12.0 kcal/mol, and the experimental value is 11.6 +/- 0.2 kcal/mol. The microcanonical RRKM rate constants, which include the effects of the variational transition state and the J rotational quantum number, are calculated. High-pressure rate constants are determined for the 223-323 K temperature range.