Users' personal citation collections reflect users' interests and thus offer great potential for personalized digital services. We studied 18,120 citations in the personal collections of 96 users of RefWorks citation management system to understand these in terms of their resolvability i.e. how well these citations can be resolved to a unique identifier and to their online sources. While fewer than 4% of citations to articles in Journals and Conferences included a DOI, we were able to increase this resolvability to 50% by using a citation resolver. A much greater percentage of book citations included an ISBN (53%), but using an online resolver found ISBNs for an additional 20% of the book citations. Considering all citation types, we were able to resolve approximately 47% of all citations to either an online source or a unique identifier.