The electronic contribution to the specific heat of NdBa <InlineEquation ID="Equ1"> <EquationSource Format="TEX">$\mathsf{_2}\mathsf{Cu_3}\mathsf{O_{6 + x}}$</EquationSource> </InlineEquation>

From measurements of the specific heat of <InlineEquation ID="Equ2"> <EquationSource Format="TEX">$\mathrm{NdBa_2Cu_3O_{6 + \mathnormal{x}}}$</EquationSource> </InlineEquation> in the temperature range between 20 K and 300 K the electronic contribution C <Subscript> e </Subscript>(T)/T has been derived. The results depend strongly on the assumptions made for the normal-state reference, especially the phonon contribution. Taking into account entropy conservation between the superconductor and a hypothetical normal-state reference, we found a temperature independent electronic contribution of this normal-state reference without any sign of a pseudogap for both optimum doped and underdoped samples. For oxygen concentrations between x=0.79 and x=0.89 a broad hump in C <Subscript> e </Subscript>(T)/T is observed around 120 K, which we ascribe to pair formation above T <Subscript> c </Subscript>. The dependence of the hole concentration n <Subscript> h </Subscript> in the copper oxide planes on the oxygen concentration x in the copper oxide chains was calculated by means of bond-valence sums. We found that the optimum doping of the copper oxide planes is n <Subscript> h,opt </Subscript>=0.24 for <InlineEquation ID="Equ3"> <EquationSource Format="TEX">$\mathrm{\mathnormal{R}Ba_2Cu_3O_{6 + \mathnormal{x}}}$</EquationSource> </InlineEquation> (R=Nd, Y) irrespective of the element on the rare-earth site. Copyright Springer-Verlag Berlin/Heidelberg 2004