Method for locating a frequency deviation in a communication network, - in which the communication network comprises a multiplicity of network nodes (S1,..., SN) which communicate with one another and each comprise an internal clock (C1,..., CN) which is clocked at a node clock frequency (fs) assigned to the respective network node (S1,..., SN), - in which synchronization messages which are used for the time synchronization of the internal clocks (C1,..., CN) of the network nodes (S1,..., SN) are transmitted in the communication network, - in which the synchronization messages are transmitted in succession from a preceding network node (S i-1) to a middle network node (S i) and, from the latter, to a next network node (S i+1) on the basis of a reference clock frequency (fm) predefined by a reference clock, the synchronization messages comprising the reference clock count state of the reference clock; - the preceding network node (S i-1), the middle network node (S i) and the next network node (S i+1) each calculate a synchronization deviation (ΔS i-1, ΔS i, ΔS i+1) at predefined intervals of time; - the middle network node (S i) carries out the following algorithm if its synchronization deviation (ΔS i) determined last exceeds a threshold value: - detecting a change in the reference clock frequency (fm) if the synchronization deviations (ΔS i-1, ΔS i, ΔS i+1) which have been calculated by the preceding network node (S i-1), the middle network node (S i) and the next network node (S i+1) are similar to one another, and otherwise - detecting a change in the node clock frequency (fs) of the middle network node (S i) if the synchronization deviations (ΔS i-1, ΔS i+1) which have been calculated by the preceding network node (S i-1) and the next network node (S i+1) are similar to one another. - in which the communication network comprises a multiplicity of network nodes (S1,..., SN) which communicate with one another and each comprise an internal clock (C1,..., CN) which is clocked at a node clock frequency (fs) assigned to the respective network node (S1,..., SN), - in which synchronization messages which are used for the time synchronization of the internal clocks (C1,..., CN) of the network nodes (S1,..., SN) are transmitted in the communication network, - in which the synchronization messages are transmitted in succession from a preceding network node (S i-1) to a middle network node (S i) and, from the latter, to a next network node (S i+1) on the basis of a reference clock frequency (fm) predefined by a reference clock, the synchronization messages comprising the reference clock count state of the reference clock; - the preceding network node (S i-1), the middle network node (S i) and the next network node (S i+1) each calculate a synchronization deviation (ΔS i-1, ΔS i, ΔS i+1) at predefined intervals of time; - the middle network node (S i) carries out the following algorithm if its synchronization deviation (ΔS i) determined last exceeds a threshold value: - detecting a change in the reference clock frequency (fm) if the synchronization deviations (ΔS i-1, ΔS i, ΔS i+1) which have been calculated by the preceding network node (S i-1), the middle network node (S i) and the next network node (S i+1) are similar to one another, and otherwise - detecting a change in the node clock frequency (fs) of the middle network node (S i) if the synchronization deviations (ΔS i-1, ΔS i+1) which have been calculated by the preceding network node (S i-1) and the next network node (S i+1) are similar to one another. - detecting a change in the reference clock frequency (fm) if the synchronization deviations (ΔS i-1, ΔS i, ΔS i+1) which have been calculated by the preceding network node (S i-1), the middle network node (S i) and the next network node (S i+1) are similar to one another, and otherwise - detecting a change in the node clock frequency (fs) of the middle network node (S i) if the synchronization deviations (ΔS i-1, ΔS i+1) which have been calculated by the preceding network node (S i-1) and the next network node (S i+1) are similar to one another.