Because of very different heating rates in hot-tool and vibration welding, and the higher weld pressures used in vibration welding inducing more squeeze flow, the weld zones in these two processes see very different flows and cooling rates, resulting in different morphologies. The weld morphologies of bisphenol-A polycarbonate (PC) and poly(butylene terephthalate) (PBT) for these two processes are discussed in relation to these differences. The thickness of the heat-affected zone (HAZ) in hot-tool welds increases with the melt time; this zone is thicker than in vibration welds. The HAZ thickness in hot-tool welds increases from the center toward the edges. The HAZ thickness is more uniform in vibration welds. Hot-tool welds of PC have large numbers of bubbles around the central plane; the bubble size increases from the center to the edges. PC vibration welds do not have bubbles except near the edges. Both hot-tool and vibration welds of PBT do not have bubbles. The morphology of the HAZ in PBT is very different in hot-tool and vibration welds. In hot-tool welds, the resolidified material consists of a sandwich structure in which two thin layers with very small crystallites surround a thicker central layer in which the spherulites are almost as large as in the original molded material. In vibration welds, the HAZ has large crystallinity gradients across the weld zone as wen as squeeze-flow induced distortion of the small spherulites.