We have determined for the first time the "electronic" specific heat coefficient y(x,T) of YBa2Cu306+x for 0.160.9. However, the continuous development of the entropy S(x,T) with x and T across the entire series suggests a progressive modification of the low energy spin spectrum with hole doping rather than a simple band model. Fermi statistics and A>space pairing are indicated by the magnitude and T dependence of S Cx, T).PACS numbers: 65.40.Em, 74.20.Mn, 74.25.Bt Fundamental questions relating to the normal and superconducting states of cuprate superconductors include the statistics of the carriers, the nature of the condensate (/c-space pairing or condensation of real space bosons), the dominant low lying excitations, and the pairing mechanism. The specific heat (C) is a bulk thermodynamic quantity determined uniquely for any material by its spectrum of excitations, and the magnitude and temperature dependence of the electronic specific heat coefficient y = C el /T provides an important test for proposed theories. Unfortunately the electronic term is only (1-2)% of the phonon term over most of the relevant temperature range and investigations of y using conventional techniques are generally limited to the vicinity of the superconducting transition (for recent reviews see Refs.[l] and [2]). Using a high resolution differential technique [3] we determined [4] from 1.8 to 300 K the difference in electronic terms between Yl^CCui-^Zn^Ov-s (0