The aqueous mixed (sodium chloride + sodium hydroxide) system has been studied using e.m.f. measurements at T = 298.15 K of the combined cells Pt\Na-Hg\NaCl(m(1)) + NaOH (m(2))\HgO\Hg\Pt and Pt\Tl-Hg\TlCl\NaCl (m(1)) + NaOH(m(2))\HgO\Hg\Pt at constant total molalities (m(1) + m(2)) of (1, 2, 3, 4, 5, 6, 7, 9, and 11) mol(.)kg(-1). The mean molal activity coefficients gamma of each single component have been determined as a function of the respective molalities. For both components, the behaviour can, with reasonable approximation, be represented by Harned's rule (linear dependence of 1ggamma on at), although a quadratic dependence is more appropriate. In terms of the mixing fraction X = m(2)/m(TOT), the 1ggamma against X curves for NaCl and NaOH have slopes in opposite directions, contrary to (KCl + KOH)(aq) and most other (MX + MY)(aq). The respective trace activity coefficients are unequal, and both of them are lower than the corresponding pure-component activity coefficients. (C) 2003 Elsevier Science Ltd. All rights reserved.