We have investigated a new type of A-B/C blend, formed between poly(methyl methacrylate-b-vinylpyrrolidone) and poly(vinylphenol) (PMMA-b-PVP/PVPh), that displays unusual phase behavior. In this blend, the PMMA (A) and PVP (B) blocks within the PMMA-b-PVP (A-B) copolymer are miscible; although PVPh (C) experiences attractive interactions (xi <= 0) through hydrogen bonding, with both the PVP and PMMA blocks, its interaction with the former block is significantly stronger than that with the latter (xi(BC) >> xi(AC)). We investigated the miscibility and phase behavior of this novel A-B/C blend through the use of FTIR spectroscopy, DSC, C-13 CP/MAS solid-state NMR spectroscopy, and TEM. The proton spin-lattice relaxation time in the rotating frame (T-1 rho(H)), which we determined using 13C NMR spectroscopy, indicates that phase separation occurs for blends containing ca. 20-60 wt % PVPh. TEM images indicated clearly that the morphology of phase separation consists of a matrix of homogeneous mixed PVP/PVPh and micellar domains of excluded PMMA. This special phase behavior and miscibility is due mainly to the diversity of interactions that exist between the PMMA/PVPh and PVP/PVPh units.