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Mixtures of Y2Si2O7 and Gd2Si2O7 were synthesized by solid-state reaction at 1600°C and characterized via in situ x-ray diffraction (XRD) to determine their coefficients of thermal expansion (CTE). All solid solutions within the system exhibited the orthorhombic δ-RE2Si2O7 (Pna21) structure. Thermal expansion measurements of Y2Si2O7 and Gd2Si2O7 correlated well with reported values in literature, and all synthesized solid solutions exhibited CTEs between Y2Si2O7 and Gd2Si2O7. Generally, there was a slight decrease in CTE exhibited by the materials with increasing Gd2Si2O7 content, with Gd2Si2O7 having the lowest CTEs and Y2Si2O7 the highest CTEs. The decrease in CTE was attributed to stronger bonds of Gd-O over Y-O, as determined by calculated crystal orbital Hamilton populations using density functional theory. However, such differences were very small and crystal structure was the dominating factor in CTE trends.