We have studied the structure and photochemistry of the formaldoxime-nitrous acid system (CH2NOH-HONO) by help of FTIR matrix isolation spectroscopy and ab initio methods. The MP2/6-311++G(2d,2p) calculations show stability of six isomeric CH2NOH center dot center dot center dot HONO complexes. The FTIR spectra evidence formation of two hydrogen bonded complexes in an argon matrix whose structures are determined by comparison of the experimental spectra with the calculated ones for the six stable complexes. In the matrix there is present the most stable cyclic complex with two O-H center dot center dot center dot N bonds; a strong bond is formed between the OH group of HONO and the N atom of CH2NOH and the weaker one between the OH group of CH2NOH and the N atom of HONO. In the other complex present in the matrix the OH group of formaldoxime is attached to the OH group of HONO forming an O-H center dot center dot center dot O bond. The irradiation Of the CH2NOH center dot center dot center dot HONO complexes with the filtered Output of the mercury lamp (lambda > 345 nm) leads to the formation of formaldoxime nitrite, CH2NONO, and its two isomeric complexes with water. The main product is the CH2NONO center dot center dot center dot H2O complex in which water is hydrogen bonded to the N atom of the C=N group. The identity of the photoproducts is confirmed by both FTIR spectroscopy and MP2 or QCISD(full) calculations with the 6-311++G(2d,2p) basis set. The intermediate in this reaction is iminoxyl radical that is formed by abstraction of hydrogen atom from formaldoxime OH group by ail OH radical originating from HONO photolysis.