In this study we investigated the properties of a novel thermophilic enzyme breaker (TEB) designed to break guar at elevated temperature. The findings were compared with the results obtained with ammonium persulfate (APS). Performance of the TEB was evaluated by two different approaches. Guar based fracturing fluids containing TEB or APS were tested in a fluid loss cell at 180degF. Berea sandstone cores were exposed to guar-based fracturing fluids containing the breaker. The filter built up on the cores’ cross-section was evaluated by determining wall building coefficients and fluid loss values. Retained permeability of the cores were determined after treatment. Whereas no significant differences in the filter built up were found after 100 min, a larger retained permeability of the core sample treated with TEB indicated a lower amount of residual broken fluid. The treated cores were examined after exposure to the fracturing fluid and shut in for 16 hours. We visualized the polymer residuals on the cores by scanning electron microscopy imaging (SEM). We obtained evidence that the use of TEB results in less permanent residue from the broken fracturing fluid compared to the APS containing system. Performance differences between TEB and APS can be explained by different breaking mechanisms