We show by STEM imaging and X-ray microanalysis that a quasiperiodic strain field and an associated composition modulation exist in InGaAsP layers LPE-grown on (001) InP. This modulation is checked not to be an artefact due to channelling effects. The composition and temperature range for which the quasiperiodic structure is observed is in good agreement with the theoretical prediction of a spinodal instability zone in the solid phase