Using differential scanning calorimetry (DSC), the heat of formation of NbAl3 was measured from the reaction of sputter-deposited Nb/Al and Nb/Al(Cu) multilayer thin-films with different bilayer thickness. The measured heat of reaction of the Nb/Al samples was 40.0+/-41 kJ/g-atom. The Nb/Al(Cu) films, with 0.5 and 1.0 wt.% Cu, yielded heats of formation of 40.8+/-0.5 and 40.5+/-0.6 kJ/g-atom, respectively. The measured heats of reaction in all three sets of samples were consistent with earlier investigations and revealed that, within experimental errors, the addition of the small amounts of Cu did not affect the formation enthalpy of NbAl3. Thus, combining the data from all three types of samples gives a value of 40.6+/-2.9 kJ/g atom for this enthalpy. Enthalpy measurements as a function of multilayer periodicity further indicated that no significant interdiffusion or intermixing took place prior to NbAl3 formation. This lack of intermixing was also confirmed by fabricating a multilayer sample with a 10 nm thick codeposited layer, having a nominal composition of Nb42Al58. interleaved between the Nb and Al layers. The presence of this intentionally intermixed layer caused a reduction in reaction enthalpy to 33.1+/-0.5 kJ/g-atom.