We have also developed our own EPR spectral simulation software for the accurate estimation of spectral linewidth from noisy multi-component EPR spectra. This is based upon a Lorentzian -Gaussian sum-function model with appropriate absorption and dispersion components. Using the special fitting program EWVoigt (Scientific Software Services) previously we have shown that the EPR line shapes of our principal oxygen sensitive probes at different oxygen concentration can be described very well by the sum of two lines with different line widths (so call two centers fitting). This approach allows us to measure the linewidth from very noisy spectra (signal to noise ratio as low as two to one) with high accuracy. Unfortunately this data processing takes time and has to be performed using a quite powerful computer. Therefore based on this result we have developed our own EPR spectral fitting software for the measurement of spectral linewidth from noisy EPR spectra. This approach is based on a sum of one Lorentzian and one Gaussian function. The algorithm also includes the fitting of absorption and dispersion components for the Lorentzian function. This spectral fitting allows us to get accurate estimation of spectral linewidth from noisy EPR spectra in a minute, even on our regular computer. This program could be improved for the spectra with wide wings by adding to the sum one more Lorentzian function.