This paper shows that not all flexible forms are equally general as has been previously assumed. An important class of flexible functional forms, when used to represent profit functions, impose rather severe prior restrictions on the production technology mainly quasi-homotheticity, certain input separability structures, and also separability between inputs and outputs. These restrictions are inherent to flexible forms which are linear on profit but do not hold for functions using nonlinear transformations of profit. These findings shed light on the important issue of discriminating among flexible functional forms based on analytical rather than purely experimental evidence.