The microstructure of a semi-continuously cast AA 6063 alloy was investigated and the transformation of insoluble intermetallic phases by applied heat treatment was studied by selective dissolution of the matrix. Residual intermetallics were characterized using energy-dispersive X-ray analysis, X-ray and electron diffraction techniques. Transformation of beta-AlFeSi phase, which is known to have detrimental effects on the surface finish of the extrudates, to more favourable alpha-AlFeSi phase by heat treatment was determined.