Initial dissolution kinetics at orthoclase (001) and (010) cleavage surfaces were measured for ∼2 to 7 monolayers as a function of temperature using in situ X-ray reflectivity. The sensitivity of X-ray reflectivity to probe mineral dissolution is discussed, including the applicability of this approach for different dissolution processes and the range of dissolution rates (∼10−12 to 10−6 mol/m2/sec) that can be measured. Measurements were performed at pH 12.9 for the (001) surface and at pH 1.1 for the (001) and (010) surfaces at temperatures between 46 and 83°C. Dissolution at pH 12.9 showed a temperature-invariant process with an apparent activation energy of 65 ± 7 kJ/mol for the (001) cleavage surface consistent with previous powder dissolution results. Dissolution at pH 1.1 of the (001) and (010) surfaces revealed a similar process for both surfaces, with apparent activation energies of 87 ± 7 and 41 ± 7 kJ/mol, respectively, but with systematic differences in the dissolution process as a function of temperature. Longer-term measurements (five monolayers) show that the initial rates reported here at acidic pH are greater than steady-state rates by a factor of 2. Apparent activation energies at acidic pH differ substantially from powder dissolution results for K-feldspar; the present results bracket the value derived from powder dissolution measurements. The difference in apparent activation energies for the (001) and (010) faces at pH 1.1 reveals an anisotropy in dissolution kinetics that depends strongly on temperature. Our results imply a projected ∼25-fold change in the ratio of dissolution rates for the (001) and (010) surfaces between 25 and 90°C. The dissolution rate of the (001) surface is higher than that of the (010) surface above 51°C and is projected to be lower below this temperature. These results indicate clearly that the kinetics and energetics of orthoclase dissolution at acidic pH depend on crystal orientation. This dependence may reflect the different manifestation of the Al-Si ordering between the T1 and T2 tetrahedral sites at these two crystal faces and can be rationalized in terms of recent theoretical models of mineral dissolution.