A process for preparing conjugated diene (co)polymers comprising polymerizing at least one conjugated diene in the presence of a catalytic system comprising: (a) at least one pyridyl iron (III) complex having general formula (I) or (II): wherein: —R1, R2, R3 and R4, identical or different, represent a hydrogen atom; or are selected from linear or branched, optionally halogenated C1-C20, preferably C1-C15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups; —R5 represents a hydrogen atom, or is selected from linear or branched, optionally halogenated C1-C20, preferably C1-C15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups; —X, identical or different, represent a halogen atom such as, for example, chlorine, bromine, iodine; or are selected from linear or branched C1-C20, preferably C1-C15, alkyl groups, —OCOR6 groups or —OR6 groups wherein R6 is selected from linear or branched C1-C20, preferably C1-C15, alkyl groups. —n is 3; (b) at least one co-catalyst selected from organo-aluminum derivatives, preferably from: (b1) aluminum compounds having general formula (III): Al(R7)(R8)(R9) (IIl) wherein R7 represents a hydrogen atom, or is selected from linear or branched C1-C20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups, alkoxy groups; R8 and R9, identical or different, are selected from linear or branched C1-C20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups; (b2) aluminoxanes having general formula (IV): (R10)2—Al—O—[-AI(R11)—O-]m-AI-(R12)2 (IV), wherein R10, R11 and R12, identical or different, represent a hydrogen atom, or a halogen atom such as chlorine, bromine, iodine, fluorine; or are selected from linear or branched C1-C20 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; (b3) partially hydrolyzed organo-aluminum derivatives; (b4) haloaluminum alkyls having general formula (V) or (VI): AI(R13)p(X′)3-p (V) AI2(R13)q(X′)3-q (VI) wherein p is 1 or 2; q is an integer ranging from 1 to 5; R13, identical or different, are selected from linear or branched C1-C20 alkyl groups; X′ represents a chlorine or bromine atom, preferably chlorine; provided that said co-catalyst (b) is not selected from organo-boron derivatives.