Significant interest has been in examining calcium phosphate ceramics, specifically -tricalcium phosphate (-TCP) (Ca-3(PO4)(2)) and synthetic hydroxyapatite (HA) (Ca-10(PO4)(6)(OH)(2)), in composites and more recently, in fibrous composites formed using the electrospinning technique for bone tissue engineering applications. Calcium phosphate ceramics are sought because they can be bone bioactive, which means an apatite forms on their surface that facilitates bonding to bone tissue, and are osteoconductive. However, studies examining the bioactivity of electrospun composites containing calcium phosphates and their corresponding osteogenic activity have been limited. In this study, electrospun composites consisting of (20/80) HA/TCP nanoceramics and poly (E-caprolactone) (PCL) were fabricated. Solvent and solvent combinations were evaluated to form scaffolds with a maximum concentration and dispersion of ceramic and pore sizes large enough for cell infiltration and tissue growth. PCL was dissolved in either methylene chloride (Composite-MC) or a combination of methylene chloride (80%) and dimethylformamide (20%; Composite-MC+DMF). Composites were evaluated in vitro for degradation, apatite formation, and osteogenic differentiation of human mesenchymal stem cells (MSCs) with an emphasis on temporal gene expression of osteogenic markers and the pluripotent gene Sox-2. Apatite formation and the osteogenic differentiation was the greatest for Composite-MC as determined by gene expression, protein production and biochemical markers, even without the presence of osteoinductive factors in the media, in comparison to Composite-MC+DMF and unfilled PCL mats. Sox-2 levels also reduced over time. The results of this study demonstrate that the solvent or solvent combination used in preparing the electrospun composite mats plays a critical role in determining their bioactivity which may, in turn, affect cell behavior. Biotechnol. Biotechnol. Bioeng. 2014;111: 1000-1017. (c) 2013 Wiley Periodicals, Inc.