A dimeric photosystem II light‐harvesting II super complex (PSII‐LHCII SC), isolated by sucrose density gradient centrifugation, was previously structurally characterized [Boekema, E. J., Hankamer, B., Bald, D., Kruip, J., Nield, J., Boonstra, A. F., Barber, J. & Rögner, M. (1995) Proc. Natl Acad. Sci. USA 92, 175−179]. This PSII‐LHCII SC bound the 33‐kDa subunit of the oxygen‐evolving complex (OEC), but lacked the 23‐kDa and 17‐kDa subunits of the OEC. Here the isolation procedure was modified by adding 1 M glycine betaine (1‐carboxy‐N,N,N‐trimethylmethanaminium hydroxide inner salt) to the sucrose gradient mixture. This procedure yielded PSII‐LHCII SC that contained both the 33‐kDa and the 23‐kDa subunits and had twice the oxygen‐evolving capacity of the super complexes lacking the 23‐kDa polypeptide. Addition of CaCl2 to PSII‐LHCII SC with the 23‐kDa subunit attached did not increase the oxygen‐evolution rate. This suggests that the 23‐kDa subunit is bound in a functional manner and is present in significant amounts. Over 5000 particle projections extracted from electron microscope images of negatively stained PSII‐LHCII SC, isolated in the presence and absence of glycine betaine, were analyzed using single‐particle image‐averaging techniques. Both the 23‐kDa and 33‐kDa subunits could be visualized in top‐view and side‐view projections. In the side view the 23‐kDa subunit is seen to protrude 0.5−1 nm further than the 33‐kDa subunit, giving the PSII particle a maximal height of 9.5 nm. Measured from the centres of the masses, the two 33‐kDa subunits associated with the dimeric PSII‐LHCII SC are separated by 6.3 nm. The corresponding distance between the two 23‐kDa subunits is 8.8 nm.