The study investigates the boundary of market efficiency and the diffusion mechanism in capital markets. A stylized information diffusion model is presented to describe the process by which investors gradually assimilate the information following an information event. The model suggests that a diffusion process depends on both a drift and a diffusion force. If prices have converged before the drift force disappears, the diffusion phenomenon is difficult to observe. Otherwise, the diffusion force dominates in the longer horizon and imposes a boundary on market efficiency. The model indicates that the speed of information diffusion depends on information content, information conductivity, and information shock absorption capacity. Each signal has a different degree of information content, and each firm has a unique information conductivity. Consequently, the information on different firms diffuses across investors at different rates. Several implications from this diffusion model are tested with quarterly earnings announcement data. The findings are consistent with an information diffusion process at work in capital markets.