We have been trying to fit experimental diffusion results to the diffusion equation using Matlab to evaluate the Diffusion coefficient. The equation we used: y=C*erfc(x/(2*sqrt(D*t))) Experimental values [x],[y] and t are given. C and D are to be evaluated from the curve fit. We used cftool of Matlab. We have a problem in getting the results. We get error message as: "Error using ==> <a href="error:C:\Program Files\MATLAB\R2007b\toolbox\curvefit\curvefit\@fittype\feval.m,97,0">fittype.feval at 97</a> Error in fittype expression ==> C.*erfc(x./(2.*sqrt(D.*300))) ??? Error using ==> erfcore Input must be real." Can anyone help?