We conducted a quasi-experiment through an online survey to investigate the effects of information security knowledge on users' adoption of security technologies. We found that knowledge can affect adoption decision in multiple ways. On the one hand, knowledge tends to increase a user's self-efficacy in using the technology, which in turn increases the perceived usefulness of the technology, consistent with the Technology Acceptance Model (TAM). On the other hand, users who are knowledgeable about information security are less likely to come up with a poor adoption decision. These findings suggest that firms may need to use different strategies when trying to market security technologies to users with different knowledge levels. Furthermore, a firm that produces useful security technologies may wish to spend resources to educate users in order to increase their self-efficacy and their perceived usefulness of the technologies.