Polyiodides (1(x)(-). x=3 and 5) and 2I(-)...I-2 adducts were established from the Raman spectra study of 1-methyl-3-propylimidazolium iodide (MPIm(+)I(x)(-) ; 1 <= x <= 5) ionic liquids containing various amounts of iodine (0mol <= I-2 <= 2 mol). The existence of I-3(-) and 2I(-)...I-2 was established for 1 <= x <= 2.5, symmetric I-3(-) ions for x=3, while linear and discrete I-5(-) was substantiated for 3 <= x <= 5. The presence of polyiodide species ill MPIm(+)I(x)(-) (1 <= x <= 5) was correlated with all enhanced ionic conductivity, attributed to the established relay-type Grotthus mechanism. Two-step conductivity increase was also reflected in decrease of the hydrogen bond interactions between the C-H ring groups and polyiodides. While in the concentration range 1 <= x <= 3 (triiodides and tetraiodides) IR bands changed only slightly in intensity, in the concentration range x > 3 the C-H stretching bands (3040-3170 cm(-1)) split and the new band at 1585 cm(-1) appeared in the IR spectra beside the already existing Im(+) ring stretching mode at 1566 cm(-1). (c) 2007 Elsevier Ltd. All rights reserved.