Summary: Forty-three natural alkali feldspars have been homogenized and used to correlate d_-201 with wt % K₂O. The regression of Or on d-₂₀₁ is Or (wt %) = 465·5 d-₂₀₁ (Å)-1877·5, and is comparable to that obtained on an exchanged orthoclase (Wright, 1968). A deviation from the Orville (1963, 1967) determinative line for synthetic sanidine-albite is greatest at the high potassium end of the series. The discrepancy is not caused by failure to homogenize or incomplete disorder. The presence of cations other than potassium and sodium, or cation deficiency, or both cannot be discounted as a cause.