The use of RE 2 Si 2 O 7 materials as environmental barrier coatings ( EBC s) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE 2 Si 2 O 7 . High‐temperature X‐ray diffraction ( HTXRD ) patterns were collected on different RE 2 Si 2 O 7 polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE 2 Si 2 O 7 compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE 2 Si 2 O 7 compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A‐ RE 2 Si 2 O 7 polymorph while the rest of polymorphs exhibited an anisotropic unit cell expansion with the biggest expansion directed along the REO x polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE 2 Si 2 O 7 compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A‐ RE 2 Si 2 O 7 compounds, with an average of 12.1 × 10 −6 K −1 , whereas the lowest values are those of β‐ and γ‐ RE 2 Si 2 O 7 , which showed average ABCTE values of ~4.0 × 10 −6 K −1 .