The electrostatic persistence lengths b(e) of NaPSS polyions have been measured as a function of ionic strength I in the solutions using a small-angle neuron scattering (SANS) technique combined with a zero average contrast (ZAG) method. This ZAC method provides the optical theta condition distinguishing the intermolecular scattering functions S-2(q), and thereby the scattering functions of a single chain S-1(q) free from the intermolecular interferences were obtained. The resulting b(e) values which were derived by the analysis of S-1(q) are proportional to I-1/2. This work confirms and extends an earlier SANS study where the contrast-match was used.