By growing (GaP) 1.5 (InP) 1.88 and (InP) 1.88 (GaP) 1.5 short-period superlattices (the former material is the first layer on the GaAs (311) substrate) by gas source MBE, composition-modulated quantum dots were self-formed in both cases. The dot size is about 20 nm in GaP/InP and 22 nm in InP/GaP sample. The photoluminescence energy is higher for the GaP/InP sample, corresponding to the difference in the dot size. The photoluminescence decay time of the InP/GaP sample is less dependent on the emission wavelength and temperature than that of the GaP/InP sample.