Spallation of environmental barrier coating (EBC) induced by thermally grown oxide (TGO) resulting from steam oxidation is a key EBC failure mode. A logical approach to improve EBC life, therefore, is to reduce TGO growth rates. A study was undertaken to investigate whether TGO growth rates can be reduced by adding modifier oxides. It was based on a hypothesis that modifier oxides dissolve in SiO2 TGO and modify the SiO2 structure, making the TGO less permeable to oxidants. Using a current state-of-the-art EBC (Si/Yb2Si2O7) as the baseline, the Yb2Si2O7 layer was modified by adding Al2O3 or Al2O3-containing oxide compounds, such as mullite and YAG (Y3Al5O12), and TiO2. EBCs were processed using air plasma spraying. Steam oxidation tests and post-oxidation test oxidation kinetics, chemistry, microstructure, and phase analysis were used to test the hypothesis. The best modified EBC reduced the TGO thickness by similar to 87% compared with that of the baseline EBC in 90% H2O+10% O-2 at 1316 degrees C under thermal cycling. Correlations between oxidation kinetics, chemistry, and microstructure of EBC and TGO were used to explain the effect of modifier oxides on reducing TGO growth rates.