Interfaces between the two phases formed in immiscible polymer pairs were studied by means of neutron reflectivity (NR) combined with small-angle neutron scattering (SANS). The interfacial widths evaluated by the two methods were compared for two systems: a nonreactive system composed of polyamide (PA) and polysulfone (PSU) and a reactive system composed of PA and PSU with a phthalic anhydride reactive end group (PSU-R) in which PSU-b-PA block copolymers can be formed at the interface. SANS measurements were made on bulk mixtures, and the NR measurements were performed on thin film bilayer stacks of PA and PSU or PSU-R. The intrinsic interfacial widths, WI,(diffuse), obtained from the two methods were compared for the samples that underwent similar annealing protocols and were quenched below the glass transition temperatures Tg's to room temperature. A consistent result for W-I,W-diffuse can be obtained for the two methods only after taking into account the following important factors: (i) the SANS intensity should be corrected for the frozen density heterogeneities within each phase as well as the frozen thermal composition fluctuations; (ii) the contribution of capillary wave fluctuations to a net (or observed) interfacial width, Wi,obs, is more significant for NR than SANS. Without the former correction, the SANS profiles at high scattering vector, q, showed asymptotic behavior of q(-n) (n < 4), giving an erroneous conclusion on WI,obs The WI,obs values obtained for the reactive system were found to be larger than those for the nonreactive system for both methods, reflecting block copolymer formation at the interfaces. The interfacial area density, , for the bulk mixture of the reactive system rapidly decreased with time and reached a constant value within ca. 2 min after annealing above the T-g's, while Sigma for the nonreactive system kept decreasing with time. This elucidates that the growth of the phase-separating domains in the reactive system was pinned by the formation of block copolymers at the interfaces during early annealing times.