Every time a user uses the Internet, a wealth of personal information is revealed, either voluntarily or involuntarily. This often causes privacy breaches, specially if the information is misused. Ideally, a user would like to make a reasoned decision about who to release her information to and what to release. For this purpose, we propose using the level of trust that a user has on the recipient regarding not to misuse her private data. To measure this trust level, we adapt the vector model of trust proposed earlier. We formalize a notion of privacy context and show how a privacy context ontology can be used to determine trust values for previously unencountered situations.