NiO and Co3O4 were prepared by thermal decomposition of the respective nitrate and mixed mechanically to give mixed oxides of any composition ('physical' mixtures). The point of zero charge (pzc) was determined by standard potentiometric titration. The surface area was measured by the BET method ('dry' area) as well as derived from titration curves using the Parsons-Zobel approach ('wet' area). The dependence of pzc on composition has been discussed in terms of mole, weight and surface area proportions, and compared with the behaviour of 'chemical' (atomically intimate) mixtures investigated previously. The surface area ratio appears to govern the pattern of the dependence of pzc on composition. (C) 2003 Elsevier B.V. All rights reserved.