Process for the preparation of polyisoprene with a mainly alternating cis -1,4- alt -3,4 structure comprising polymerizing isoprene in the presence of a catalytic system comprising: (a) at least one pyridyl iron complex having general formula: wherein: - R 1 is selected from linear or branched C 1 -C 20, preferably C 1 -C 15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups; - R 2 is selected from linear or branched C 1 -C 10, preferably C 1 -C 3, alkyl groups; - X, mutually identical or different, represent a halogen atom such as chlorine, bromine, iodine; or they are selected from linear or branched C 1 -C 20 preferably C 1 -C 15, alkyl groups, -OCOR 3 groups or -OR 3 groups wherein R 3 is selected from linear or branched C 1 -C 20 preferably C 1 -C 15, alkyl groups; - n is 2 or 3; (b) at least one co-catalyst selected from organo-derivative compounds of aluminum, preferably from: (b 1) aluminoxanes having general formula: (R 4) 2 -Al-O-[-Al(R 5)-O-] m -Al-(R 6) 2 wherein R 4, R 5 and R 6, mutually identical or different, represent a hydrogen atom, or a halogen atom such as chlorine, bromine, iodine, fluorine; or they are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, said groups being optionally substituted with one or more silicon or germanium atoms; and m is an integer ranging from 0 to 1000; (b 2) aluminum compounds having general formula: Al(R 7)(R 8)(R 9) wherein R 7 represents a hydrogen atom, or is selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups, alkoxy groups; R 8 and R 9, mutually identical or different, are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups; wherein the molar ratio between the aluminum present in the co-catalyst and the iron present in the pyridyl iron complex having general formula is ranging from 5 to 60, preferably ranging from 8 to 55. (a) at least one pyridyl iron complex having general formula: wherein: - R 1 is selected from linear or branched C 1 -C 20, preferably C 1 -C 15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups; - R 2 is selected from linear or branched C 1 -C 10, preferably C 1 -C 3, alkyl groups; - X, mutually identical or different, represent a halogen atom such as chlorine, bromine, iodine; or they are selected from linear or branched C 1 -C 20 preferably C 1 -C 15, alkyl groups, -OCOR 3 groups or -OR 3 groups wherein R 3 is selected from linear or branched C 1 -C 20 preferably C 1 -C 15, alkyl groups; - n is 2 or 3; - R 1 is selected from linear or branched C 1 -C 20, preferably C 1 -C 15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups; - R 2 is selected from linear or branched C 1 -C 10, preferably C 1 -C 3, alkyl groups; - X, mutually identical or different, represent a halogen atom such as chlorine, bromine, iodine; or they are selected from linear or branched C 1 -C 20 preferably C 1 -C 15, alkyl groups, -OCOR 3 groups or -OR 3 groups wherein R 3 is selected from linear or branched C 1 -C 20 preferably C 1 -C 15, alkyl groups; - n is 2 or 3; (b) at least one co-catalyst selected from organo-derivative compounds of aluminum, preferably from: (b 1) aluminoxanes having general formula: (R 4) 2 -Al-O-[-Al(R 5)-O-] m -Al-(R 6) 2 wherein R 4, R 5 and R 6, mutually identical or different, represent a hydrogen atom, or a halogen atom such as chlorine, bromine, iodine, fluorine; or they are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, said groups being optionally substituted with one or more silicon or germanium atoms; and m is an integer ranging from 0 to 1000; (b 2) aluminum compounds having general formula: Al(R 7)(R 8)(R 9) wherein R 7 represents a hydrogen atom, or is selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups, alkoxy groups; R 8 and R 9, mutually identical or different, are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups; wherein the molar ratio between the aluminum present in the co-catalyst and the iron present in the pyridyl iron complex having general formula is ranging from 5 to 60, preferably ranging from 8 to 55. (b 1) aluminoxanes having general formula: (R 4) 2 -Al-O-[-Al(R 5)-O-] m -Al-(R 6) 2 wherein R 4, R 5 and R 6, mutually identical or different, represent a hydrogen atom, or a halogen atom such as chlorine, bromine, iodine, fluorine; or they are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, said groups being optionally substituted with one or more silicon or germanium atoms; and m is an integer ranging from 0 to 1000; (b 2) aluminum compounds having general formula: Al(R 7)(R 8)(R 9) wherein R 7 represents a hydrogen atom, or is selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups, alkoxy groups; R 8 and R 9, mutually identical or different, are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups;