The model studies information sharing and the stability of cooperation in cost reducing Research Joint Ventures (RJVs). In a three-stage game-theoretic framework, firms decide on participation in a RJV, information sharing along with R&D expenditures, and output. An important feature of the model is that voluntary information sharing between cooperating firms increases information leakage from the RJV to outsiders. It is found that RJVs representing a larger portion of firms in the industry are more likely to share information. It is also found that when sharing information is costless, firms generally don' t choose intermediate levels of information sharing: they share all the information or none at all. The size of the RJV is found to depend on three effects: a coordination effect, an information sharing effect, and a competition effect. Depending on the relative magnitudes of these effects, the size of the RJV may increase or decrease with spillovers. In response to an increase in leakages, RJV members reduce their R&D spending. In addition, they either increase the RJV size while maintaining information sharing unchanged (when leakages are low), or they reduce both information sharing and RJV size (when leakages are high). The effect of information sharing on the profitability of firms as well as on welfare is studied.