Tilt grain boundaries in poly(styrene-b-butadiene)lamellar diblock copolymers were characterized using transmission electron microscopy (TEM). Three distinct tilt grain boundary morphologies were observed : the chevron, the omega boundary, and the T-junction. All three boundary types are found to have intermaterial dividing surfaces, separating the polystyrene and polybutadiene microphases, that approximate surfaces of constant mean curvature. The chevron and omega morphologies are observed for symmetric tilt boundaries, and a transition occurs from the chevron to the omega as tilt is increased at constant boundary width. A transition from either the chevron or the omega boundary to the T-junction occurs as the boundary becomes more asymmetric. Analyses of both of these transitions using geometrical arguments are in agreement with experimental observations.