Ab initio infrared and Raman phonon modes in the normal cubic MgAl₂O₄ spinel are calculated at the first Brillouin zone centre using density functional theory with plane-wave basis and norm-conserving pseudopotentials. Good agreement with available experimental infrared and Raman spectra is found for both natural and synthetic spinel. A new set of infrared measurements on synthetic spinels shows extra vibrational infrared modes which are not predicted by calculations on normal spinel. The ionic character of spinels is illustrated by comparison between calculated and experimental Born effective charges.