Purpose: To measure inaccuracies on T1 qMRI caused by magnetization transfer effects that are inherent to MRI pulse sequences. Methods: The mixed-TSE pulse sequence was used at several RF power levels to image a series of agarose gels, sucrose solutions, and Gd solutions. Results: Measured T1s are increasingly underestimated as a function of increasing semisolid pool size and RF power level. Conclusion: Uncorrected magnetization transfer effects can cause large underestimation of T1 measurements in tissue imaged with RF intensive pulse sequences. This work could have implications for the design of more accurate qT1 mapping algorithms.