Elastic constants and their temperature derivatives of orthorhombic 2/1-mullite single crystals were determined using resonant ultrasound spectroscopy between ambient temperature and 1400 degrees C in air. The values at room temperature given in GPa were c(11)=279.5(4), c(22)=234.9(2), c(33)=360.6(3), c(44)=109.49(7), c(55)=74.94(4), c(66)=79.89(4), c(12)=103.1(2), c(13)=96.1(3), and c(23)=135.6(1). The high temperature ultrasound data sets were corrected for thermal expansion effects using the coefficients, which were measured in the range -170 degrees to 1400 degrees C by means of dilatometry. Up to about 1000 degrees C, the temperature evolution of the elastic stiffnesses was linear and the corresponding thermoelastic constants T-ij=d log c(ij)/dT given in 10(-6)degrees C-1 were T-11=-100(2), T-22=-126(2), T-33=-78.2(6), T-44=-75.3(7), T-55=-81.3(9), T-66=-113(1), T-12=-153(3), T-13=-94(4), and T-23=-47(2). While the temperature derivatives of the longitudinal stiffnesses and the transverse interaction coefficients remained linear, the shear resistances c(44), c(55), and c(66) displayed an increased softening above about 1000 degrees C, which was accompanied by a rapidly increasing ultrasound dissipation. In the same temperature region, small, but clearly recognizable discontinuities in the longitudinal and in the volume thermal expansion coefficients occurred. Taking into account the behavior of the heat capacity, the observed anomalies may indicate a glass-like transition of mullite above about 1000 degrees C. An unexpected peak in the ultrasound dissipation at about 70 degrees C was most likely caused by anelastic point defect relaxations.