Optical properties of GaN/AIGaN single quantum well (SQW) and multi quantum well (MQW) structures grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire have been characterised by low temperature photoluminescence. Photoluminescence (PL) peaks corresponding to emission from very narrow (10 Å wide) SQWs are blue shifted with respect to the bulk GaN emissions but reveal strong red shift for wider SQWs (40 and 60 Å wide). Structural properties of SQW and MQW structures have been characterised by X-ray reciprocal lattice mapping in order to determine the strain conditions and composition of ternary A1GaN alloys. The results clearly demonstrate that in all structures under investigation the ternary (barrier) alloys are fully strained to underlying MOCVD-grown GaN/sapphire substrates. Despite of the fact that GaN quantum wells (QW) are unstrained, photoluminescence experiments clearly reveal the presence of extremely strong electric fields in the QW regions, which can be attributed to the interplay of the piezoelectric-type polarisation in the well and barrier layers due to Fermi level alignment.