Scott, For many cases, I think it makes very little difference if you first fit the first shell and then fit the second shell keeping "the first shell" fixed. That is, for many cases, the two are nearly completely separated. If they are not well separated, then you pretty much have to fit them together. But many EXAFS problems do have well separated shells, often with the first shell much larger, of a known atom type, and at a pretty well-known distance. A fairly typical class of EXAFS analysis would have oxygen as the first shell and a metal as the second shell and the goals are 1) is the oxygen distance / coordination of type A or type B (say, tetrahedral v. octahedral). The XANES often tells you this, and the EXAFS signatures (N, R) are pretty distinct. It may not be important how distorted this coordination is: one just wants the average, gross coordination type. 2) what is the second coordination shell? What atom type, what coordination number, what distance? There is often a much wider range of possibilities that are harder to distinguish, and this is important to study (fit) in detail. In these cases, many people use the first shell (again, often well isolated, large, and of a known atom type at a pretty well known distance) to determine S02 and E0. This can often be done in a few minutes. With that information, higher shells can be tackled adjusting atom type, number, distance, and disorder, and can take a long time to do. In many of these cases, leakage from the first shell just does not matter much, and one does not need (or have) constraints or restraints between parameters affecting the separate shells. In these cases, It is generally better to tell the fit to look only at the higher shell so that the inherent misfit in the first shell (inevitable, and generally of little importance to the result) does not dominate the total fit uncertainty. In short, it speeds up the process (both for the computer and the human) to fit only the second shell. I would say that this approach is not only "not wrong", it is a completely valid and recommended. In the end, it is a good idea to fit to the entire spectra as there can be subtle correlations between the parameters. If this step is forgotten, it hardly invalidates the analysis, but it is definitely better to do. --Matt