  Bounds on the shape parameters of K_{l3} form factors are investigated in a
general model independent framework. Using the method of unitarity bounds for
correlators evaluated in pQCD, chiral symmetry and experimental information on
the phase and modulus of \pi K form factors up to an energy t_{in}=1 GeV^2, we
constrain the shape parameters of the scalar and vector form factors, the value
of the scalar form factor at t=M_\pi^2-M_K^2, and exclude regions where zeros
can exist on the real energy line and in the complex energy plane.
