III-Nitride bandgap and refractive index data are of direct relevance for the design of (In, Ga, Al) N- based photonic and electronic devices. The bandgaps and bandgap bowing parameters of IIInitrides across the full composition range are reviewed with a special emphasis on InxAl1-xN, where less consensus was reached in the literature previously. Considering the available InAlN data, including those recently reported for low indium contents, empirical formulae for InAlN bandgap and bandgap bowing parameter are proposed. Applying the generalised bandgap data, the refractive index dispersion data available in the literature for III-N alloys is fitted using the Adachi model. For this purpose, a formalism involving a parabolic dependence of the Adachi parameters on the dimensionless bandgap xi(Eg) = (E-g,A(x)B(1-x)N - E-g,E-BN)/(E-g,E-AN - E-g,E-BN) of the corresponding ternary alloys is used rather than one directly invoking the alloy composition.