Live chat, which is embedded in some shopping websites, allows a retailer to communicate with its shoppers directly and respond to their inquiries promptly. We extended the Speech Act Theory (SAT) to this context by developing the Dialog Conversion Model (DCM) that elucidates the characteristics of online dialog between shoppers and retailers. We validated the DCM using quantitative and qualitative data. The quantitative analyses of 1,185 communication sessions and related purchase records reflect that shoppers' cognitive, rather than affective, expressions tend to indicate their purchase intentions, and retailers should emphasize cognitive expressions to the extent possible while being strategic in their choice of expressions. Furthermore, the affective expressions (in positive and negative forms) articulated by retailers have a considerable influence on purchase conversion. However, the same is not true for shoppers. Post-hoc interviews were conducted with shoppers to gain additional insights.