Hello Consuelo, I understand that you have the electrical impedance (R + jX) measurements for various frequencies for a series resistor and capacitor. If this is the case, you should be able to directly calculate the resistance and capacitance parameters (R will not change [much] with frequency and C = -1/(w*X), where "w" is frequency in radians/sec).
Further, if you have the transfer function corresponding to your measurements, you can calculate the theorietical transfer function for a series R-C circuit with the output as the terminal voltage and in the input as the current. Once again, you can calculate R and C.
If I have misunderstood you and the situation is more complex, I think you will benefit the System Design Optimization Toolbox. It includes the ability to estimate parameters from a system model and experimental measurements. The following examples hows one way to do this.
