The full elastic tensors of two K-rich monoclinic alkali feldspars, Or83Ab15 sanidine and Or93Ab7 orthoclase, have been determined by using the Impulse Stimulated Light Scattering technique to measure surface acoustic wave velocities. The new data confirm that alkali feldspars exhibit extreme elastic anisotropy, so the bounds of their isotropic average properties span a wide range. The measured adiabatic moduli are, for Or83Ab15 and Or93Ab7, respectively, KReuss = 54.7(7), 54.5(5) GPa; KVoigt = 62.9(1.1), 64.4(0.6) GPa; GReuss = 24.1(1), 24.5(1) GPa; and GVoigt = 36.1(5), 36.1(7) GPa. The small differences in moduli between the samples suggests that variations in composition and in state of Al, Si order only have minor effects on the average elastic properties of K-rich feldspars. The new measurements confirm that the earliest determinations of elastic wave velocities of alkali feldspars, widely used to calculate wave velocities in rocks, resulted in velocities systematically and significantly too slow by 10% or more.