The diorganodiselenides (pzCH 2 CH 2 ) 2 Se 2 (1) and (PhtzCH 2 ) 2 Se 2 (2) were prepared by reacting Na 2 Se 2 with 1-(2-bromoethyl)-1H-pyrazole and 4-(chloromethyl)-2-phenylthiazole, respectively, while the reactions between 1-(2-bromoethyl)-1H-pyrazole or 4-(chloromethyl)-2-phenylthiazole and the lithium organoselenolates [2-(Et 2 NCH 2 )C 6 H 4 ]SeLi and [2-{O(CH 2 CH 2 ) 2 NCH 2 } C 6 H 4 ]SeLi in a 1:1 molar ratio resulted in the heteroleptic diorganoselenium(II) compounds [2-(Et 2 NCH 2 )C 6 H 4 ](R)Se (R = pzCH 2 CH 2 (3) or PhtzCH 2 (5)) and [2-{O(CH 2 CH 2 ) 2 NCH 2 }C 6 H 4 ](R)Se (R = pzCH 2 CH 2 (4) or PhtzCH 2 (6)). The diorganotin(IV) bis(organoselenolato) derivatives of type R 2 Sn(SeCH 2 CH 2 pz) 2 (R = 2-(Me 2 NCH 2 )C 6 H 4 (7) or Me (8)) were obtained by reacting (pzCH 2 CH 2 ) SeNa with the appropriate diorganotin(IV)dichloride in a 2:1 molar ratio. All compounds were investigated using NMR spectroscopy ( 1 H, 13 C, 77 Se, 119 Sn as appropriate) and ESI+ mass spectrometry. The molecular structures of 2 and 6 were determined using single-crystal X-ray diffraction. The formation of a 10-Se-3 hypercoordinated species was evidenced for 6 in the solid state, as a consequence of the C,N coordination behaviour of the 2-{O(CH 2 CH 2 ) 2 NCH 2 } C 6 H 4 group. Compounds 1, 7 and 8 were investigated for their antiproliferative activity towards the mouse colon carcinoma C26 cell line with the preliminary results showing a better activity than 5-fluorouracil.