The ring-opening metathesis polymerization (ROMP) of functional norbornenes, i.e., exo,endo-5-norbornene-2-carbonitrile (3), methyl (exo,endo-5-norbornene-2-carboxylate) (4), norborn-5-ene-2-yl acetate (5), 2-propyl exo,endo-5-norbornene-2-carboxylate (6), norborn-5-ene-2-carboxylic acid (7), exo,endo-5-norbornene-2-methanol (8), exo,endo-5-norbornene-2-ylmethyl bromide (9), exo,endo-5-norbornene-2-ylmethylamine (10), exo,endo-5-norbornene-2-triethoxysilane (11), dimethyl exo,endo-5-norbornene-2-yl phosphonate (12), exo,endo-5-norbornene-2-ylcarboxyethyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide (13), exo,endo-5-norbornene-2-ylcarboxyethyl-3-ethylimidazolium tetrafluoroborate (14), 7-oxanorborn-5-ene-2,3-dicarboxylic anhydride (15), was performed in a variety of pure ionic liquids (ILs) in the absence of any cosolvent. Both imidazolium, i.e., [1-butyl-3-methylIM]+X- and [1-butyl-2,3-dimethylIM]+X- (IM+ = imidazolium, X- = PF6-, CF3SO3-, (CF3SO2)2N-, CF3COO-, NO3-, BF4-, Br-), and phosphonium-based ILs, i.e., [P+(C6H13)3(C14H29)]X- (X- = PF6-, BF4-, Cl-), were used. In this context, the principal compatibility of an IL with a series of ruthenium-based catalysts, i.e. RuCl2Py2(IMesH2)(CHPh) (1) and RuCl2(IMesH2)(2-(2-PrO−C6H4) (2) (Py = pyridine, IMesH2 = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazolin-2-ylidene) was shown. The influence of temperature, concentration of both the initiator and the monomer and nature of the IL on ROMP was investigated. ROMP of norbornene derivatives in ionic solvents proceeded with high speed and offered access to high molecular weight polymers (Mw up to 1 500 000 g/mol) in high yields. Most important, ILs allowed for the synthesis of polymers from monomers that are hardly polymerizable in organic solvents, e.g. of 15. The use of 3 in [1-methyl-3-butylIM]+PF6- as the reaction medium along with 2-(2-propoxy)styrene allowed for the recycling of the ruthenium catalyst. The interaction of catalyst 2 with different ILs, which allowed for an explanation of the influence of the IL's nature upon the polymerization process, was investigated by IR- and UV−vis spectroscopy. These investigations will facilitate the choice of the optimum IL in future investigations.