Microphase separation in poly(acrylonitrile-butadiene-styrene) (ABS) was studied as a function of the butadiene content and method of preparation with electron spin resonance (ESR) spectra of nitroxide spin probes. Results for the ABS polymers were evaluated by comparison with similar studies of the homopolymers polybutadiene (PB), polystyrene (PS), and polyacrylonitrile (PAN) and the copolymers poly(styrene-co-acrylonitrile) (SAN) and poly(styrene-co-butadiene) (SB). Two spin probes were selected for this study: 10-doxylnonadecane (10DND) and 5-doxyldecane (5DD). The probes varied in size and were selected because their hydrocarbon backbone made them compatible with the polymers studied. The ESR spectra were measured in the temperature range 120-420 K and were analyzed in terms of line shapes, line widths, and hyperfine splitting from the N-14 nucleus; the appearance of more than one spectral component was taken as an indication of microphase separation. Only one spectral component was detected for 10DND in PB, PS, and PAN and in the copolymers SAN and SB. In contrast, two spectral components differing in their dynamic properties were detected for both probes in the three types of ABS samples studied and were assigned to spin probes located in butadiene-rich domains (the fast component) and SAN-rich domains (the slow component). The behavior of the fast component in ABS prepared by mass polymerization suggested that the low-T-g (glass-transition-temperature) phase was almost pure PB. The corresponding phase in ABS prepared by emulsion grafting also contained styrene and acrylonitrile monomers. A redistribution of the spin probes on heating occurred with heating near the T-g of the SAN phase, suggesting that the ABS polymers as prepared were not in thermodynamic equilibrium.