The MOVPE growth of Al0.3Ga0.7As over trenches is compared to the growth of its binary components GaAs and AlAs. The growth rates for GaAs, AlAs and AlGaAs on planar (1 0 0), (3 1 1)A and (3 1 1)B substrates are independent of orientation and the AlGaAs growth rate and composition can be described by the sum of the two binaries. For growth over trenches with {3 1 1}-sidewalls the growth rate and the composition on the sidewalls are different compared to (100) due to interplay of the adjacent facets. The AlAs growth rate is nearly the same on the sidewalls and the planar regions indicating that Al is incorporated without extensive surface diffusion. For growth temperatures above 660 degrees C the GaAs growth rate on the sidewalls is enhanced indicating strong Ga diffusion from (1 0 0) to the sidewalls. From these two binary growth rates the Al0.3Ga0.7As growth rate and composition is calculated and compared to the measured values. On the sidewalls the measured AlGaAs growth rate is lower and the Al content is higher than expected from the binary results.