Abstract Coarsening of a dendritic Al–10 wt.% Cu alloy at 570 °C was investigated by fast in situ X-ray tomography. The variation of the specific solid–liquid interface area as a function of time was found to follow a power-law relation with an exponent close to −1/4. Three coarsening mechanisms operate in the alloy: (i) melting of small dendrite arms to the benefit of the adjacent arms (small arm melting), similar to Ostwald ripening; (ii) gradual movement of the base of the interdendritic grooves towards the tips (interdendritic groove advancement); (iii) coalescence of dendrites near the tips leading to the entrapment of liquid tubes or droplets (coalescence and groove advancement). The coarsening models proposed in the literature do not adequately resemble or predict the behaviour observed in the experiments. New models were developed for the first two coarsening mechanisms and predict the coarsening rate with very good agreement.