Chemical-deposition of nanocrystalline PbS, CdS, and ZnS at the air/solution interface in the absence and presence of a polydiacetylene (PDA) Langmuir film is investigated in situ using grazing incidence X-ray diffraction. In all cases it is found that PDA has a pronounced effect on the incipient semiconductor nanocrystals (NCs). In the presence of PDA, PbS NCs showed a < 111 > orientation in addition to the commonly obtained < 100 > growth direction of the PbS rock salt structure while CdS and ZnS NCs crystallized in the zinc blende polymorph with a predominant < 100 > orientation ZnS NCs were obtained only in the presence of PDA at the air/solution interface.