The complex permeability and permittivity of the constituent materials of the microwave absorbers play a key role in determining the reflection or attenuation properties of the incident microwave. The real and imaginary (loss) components of mu(r) and epsilon(r) in the frequency range 1-12 GHz, and their relationship with microwave-absorbing properties, have been investigated in the NiZnCo ferrite composites. The composite specimens were prepared by moulding and curing a mixture of prereacted ferrite powder and silicone rubber. The quantitative estimation of microwave-absorbing properties was made by plotting the observed material constants (mu(r), epsilon(r)) on the calculated solution map of impedance-matching. It was found that the CoO content influences the first matching frequency and the first matching thickness in the NiZnCo ferrite composites, but does not influence the second matching frequency and second matching thickness.