Abstract: The new method proposed by Ferguson (1980, 1981) for estimating the T-site occupancies in K-rich feldspars from lattice constants is discussed and compared with the linear model using the structural indicators Δ(bc), Δ(b*c*), Δ(αγ) Δ(α*γ*). The latter method appears to be considerably more effective in view of its wider application to alkali feldspars of different compositions and structural states. Ferguson's (1980, 1981) method has been conceived for K-rich feldspars alone and may produce results that tend to be more erroneous the further a K-rich feldspar lies from the two-step ordering paths involved in the formulation of the determinative procedure itself.