  Thermodynamics is derived rigorously for the 1D supersymmetric {\it t-J}
model and its SU($K,1$) generalization with inverse-square exchange. The system
at low temperature is described in terms of spinons, antispinons, holons and
antiholons obeying fractional statistics. They are all free and make the spin
susceptibility independent of electron density, and the charge susceptibility
independent of magnetization. Thermal spin excitations responsible for the
entropy of the SU($K,1$) model are ascribed to free para-fermions of order
$K-1$.
