The critical current density, J(c), in fluorine-doped YBa2Cu3Ox bulk superconductors has been measured at several temperatures. The measured J(c) value is 10(8) A m(-2) at 77 K and 0.5 T. A sample with a larger critical current density can be produced by the addition of a small amount of platinum during the sintering process. The added platinum has the effect of evenly distributing Y2BaCuO5 particles and voids in the matrix in comparison with the undoped sample. These distributions of Y2BaCuO5 particles and voids are reflected in an increase in the critical current density. However, these distributions are not directly related to the observed peak effect in the critical current density. If the fluorine-doped YBa2Cu3Ox superconductor consists of the matrix and a phase with a Ginzburg-Landau parameter, that is slightly different from that of the matrix, then the occurrence of the peak effect can be explained.