The magnetization curves of CoFe2O4 ferrofluids, p-NiFe2O4 paramagnetic fluids and CoFe2O4–p-NiFe2O4 binary ferrofluids, in which the volume fraction of CoFe2O4 particles φCo is 0.6% and one of p-NiFe2O4 particles φNi is 0.2%, 0.4%, 0.6% and 0.8% respectively, prepared by the Massart method, have been measured at room temperature. Comparison of the experimental data from the CoFe2O4 ferrofluids with the Langevin theory curves demonstrates a considerable difference between them, but a curve fitted using a model of a gas-like compression (MGC) agrees with the experimental data very well. The experimental results show that the magnetization of the CoFe2O4–p-NiFe2O4 binary ferrofluid is not a simple summation of the ferrimagnetic CoFe2O4 part and the paramagnetic p-NiFe2O4 part. From the fitted results, it was found that the saturation magnetization of the CoFe2O4 part of the binary ferrofluid depends non-monotonically on the p-NiFe2O4 particle volume fraction, and the CoFe2O4 part is a stronger “hard” magnet than CoFe2O4 in simple ferrofluids. The magnetization behavior of the binary ferrofluids is explained by the modification of the microstructure of CoFe2O4 nanoparticle system by the p-NiFe2O4 nanoparticle system.