The nematic cylindrical (Nc)−nematic discotic (Nd) phase transitions in three amphiphile/decanol/water systems, occurring as a function of the decanol/amphiphile molecular ratio (M d), are correlated with a change of micellar form from spherocylinder (SC) to square tablet (ST), which occurs geometrically in a continuous way, with an intermediate biaxial object. The relative stability of these forms is measured by the micellar elastic bending energy, calculated in the surfactant parameter model, as a function of the molecular quantity p = v/al (where v, l, and a are respectively the volume and length of the hydrophobic chain and the area per polar head at the apolar−polar interface) and of the decanol/amphiphile mixing. A SC−ST transition is predicted for increasing M d and p 0 values, because the amphiphile (spontaneous p 01) and decanol (p 02) molecules prefer zones with different curvatures. A good agreement is obtained by comparison of the predicted SC−ST transformation with experimental results obtained at the Nc−Nd transitions for KL (potassium laurate), SDS (sodium decyl sulfate), and SLS (sodium dodecyl sulfate).