The surface of ZnS and PbS has been modified by interfacing PbS on ZnS and ZnS on PbS nanoparticles. This produced core-shell nanocomposites ZnS/PbS and PbS/ZnS with tunable electronic properties. In both structures PbS particles are present in cubic form with all average diameter of about 6 nm. The addition of Pb2+ (>= 3 x 10(-4) mol dm(-3)) to Q-ZnS (1.5 x 10(-4) mol cm(-3)) in the basic pH range produces size-quantized fluorescent PbS particles coated by metal hydroxides. In these particles the relaxation kinetics of charge carriers has been followed using a picosecond single-photon counting technique. At <= 1.5 x 10(-4) mol dm(-3) Pb2+ an interfacial relaxation of charge from ZnS to PbS phase could be observed in subnanosecond time domain. An increase in [Pb2+] from 2 x 10(-4) to 1 x 10(-3) mol dm(-3) enhanced the average emission lifetime florin 9.4 to 19.4 ns. Composite PbS/ZnS particles are produced at high [ZnS] only. These particles had emission lifetime in mu s time range. The extent of charge separation and the dynamics of charge carriers could be manipulated by the surface modification of these nanostructures. (c) 2005 Elsevier Inc. All rights reserved.