In thrombotic events and diseases such as cancer, HIV/AIDS, dysfibrinogenaemia, as well as acute incidents (e.g. burn wounds), ultrastructure of platelets and fibrin networks change. In the current study, we compare the ultrastructure of platelets and fibrin networks of apheresis platelets stored in citrated human plasma (CP) and in a first-generation platelet additive solution (PAS) (T-Sol), to that of fresh donor plasma (FP). Eighteen apheresis platelet donors donated platelets on Trima®-Accel™ V5.2 and V5.1 cell separators. Six collections were stored for five days in autologous citrated plasma (CP); six collections were stored in 40% citrated human plasma and 60% PAS solution (CP/PAS) controlled, for the duration of storage, at a constant temperature (22±2°C) with continuous flat-bed agitation; and six collections were stored in conditions uncontrolled for temperature and without continuous agitation. On days 1, 3 and 5, equal volumes of human thrombin were mixed with platelets collected in either CP or CP/PAS to form a coagulum (fibrin network containing platelet aggregates), followed by preparation for scanning electron microscopy. Results were compared with platelets and fibrin networks in FP. Typically, in FP, platelet aggregates with smooth membranes and pseudopodia are seen and fibrin networks arrange to form major, thick fibers and scattered, minor, thin fibers. On day 1, in CP and in all CP/PAS units, platelet ultrastructure compared well to that of FP, although the fibrin fibers were denser, with the minor fibers forming a matted layer over the major fibers. On day 3, in platelet units uncontrolled for temperature and without continuous agitation during storage, some platelet aggregates in CP/PAS showed typical apoptotic morphology, with shrinkage and membrane damage, but comparable fibrin networks were present. On day 5 however, in those units where storage conditions were uncontrolled and where the pH had decreased to below 6.4, no platelet aggregates were seen and fibrin was arranged into short, lumpy masses with no separate major or minor fibrin fibers visible. In those units stored at 22°C with continuous flat-bed agitation, where pH was maintained >7.0, ultrastructure of platelets and fibrin network in CP/PAS was typical and similar to FP and CP at the end of five days of storage. Examining platelet and fibrin network ultrastructure may be useful, in addition to conventional laboratory analysis, in assessing the viability and potential clinical efficacy of platelets for transfusion and could play a role in the evaluation of new generation platelet additive solutions.