This paper analyzes the process through which more than 650,000 off-grid households in rural Bangladesh decided to purchase a solar home system (SHS) from 1996 to 2010. Our hypothesis was that positive word of mouth is the primary driver of these sales. We tested the hypothesized diffusion process through a combination of semi-structured key informant interviews and an examination of 100 households in Panchua Village, including 60 with a SHS and 40 without a system. The data from Panchua Village suggest that the hypothesis was correct, although they do not explain why owner’s influenced others to purchase a system. Our analysis of this process addsto the technology diffusion literature by highlighting the role of opinion leaders in SHS diffusion and quantitatively testing the role that word of mouth played in driving SHS sales.