The critical slab problem based on the singular eigenfunction method solution is generalized using one-speed neutron transport theory. Changing vacuum boundary to reflective boundary, a slab is investigated to rederive the criticality conditions when both reflection coefficients are the same in both surfaces. A special emphasis is given to the evaluation of the eigenvalue spectrum and the results are presented. It is also shown that there exist several critical thicknesses corresponding to each eigenvalue. The results are compared with data available in the literature.