In the business-to-business (B2B) sector, new electronic commerce (e-commerce) initiatives like Internet-enabled supply chains and electronic marketplaces (e-marketplaces) offer firms significantly lower procurement costs, increased operating efficiencies, and expanded market opportunities. Using an event-study methodology, we find that the capital markets respond favorably to firms announcing new B2B e-commerce initiatives and alliances. For B2B e-marketplaces, we find slightly higher, though statistically insignificant, average abnormal returns associated with vertical markets than with horizontal markets. When examining the data by the type of partner the e-commerce provider aligns with, we find that the capital markets reward firms the greatest when they form alliances with a competitor or a computer industry giant. The abnormal returns associated with these announcements are on average more than three times greater than returns from announcing a B2B e-commerce initiative alone or with Old Economy industry leaders.