Hydrolysis of polyacrylonitrile (PAN) was carried out in 65% HNO3 at 21 degreesC. The composition and structure of hydrolyzed samples (HPAN) were determined using C-13 NMR spectrometry. The HPAN are multiblock copolymers containing acrylonitrile (A) and acrylamide (B) units. A set of individual rate constants for the reaction of central A units in AAA, AAB + BAA, and BAB triads, k(0), k(1), and k(2), respectively, describes fairly well both the reaction kinetics and the HPAN distribution of units. Thus, the accelerating effect of neighboring groups (k(0):k(1):k(2) = 1:18:36) is the principal, if not exclusive, factor determining the peculiarities of the acidic PAN hydrolysis. Less than 3% of acrylamide groups are involved in cyclization. (formation of glutarimide cycles) under the hydrolysis conditions used, so that cyclization may be neglected in the analysis of the hydrolysis kinetics and the HPAN structure within the framework of the neighbor effect.