A Mazur space is a locally convex topological vector space X such that every f ε X^s is continuous where X^s is the set of sequentially continuous linear functionals on X; X^s is studied when X is of the form C(H), H a topological space, and when X is the weak * dual of a locally convex space. This leads to a new classification of compact T_2 spaces H, those for which the weak * dual of C(H) is a Mazur space. An open question about Banach spaces with weak * sequentially compact dual ball is settled: the dual space need not be Mazur.