A method of measuring an amount of loss of material thickness from a solid structure operable to support propagation of acoustic waves therein, said structure being in operation in contact with one or more substances susceptible to changing a thickness of the structure, whereby said method employs a plurality of acoustic transducers of any type arranged in contact with a surface of said solid structure and a processing unit connected to said transducers, whereby said method comprises a measurement procedure of; (a) transmitting an acoustic Lamb wave signal from at least one of said acoustic transducers into a wall of said structure and receiving the acoustic signal at a plurality of said transducers after propagating a distance within the solid; (b) collecting signals from said plurality of transducers in processing unit; (c) repeating steps (a) and (b), transmitting and receiving from said plurality of transducers; and (d) analysing the received signals generated in steps (a) - (c); (e) analysing a group velocity and a phase velocity of said Lamb wave propagation mode of said signal with the processing unit; and (f) computing a change in a true thickness of said solid structure; characterized in the following steps, the signal received by the receiving acoustic transducer and analyzed in the processing unit is a fundamental flexural mode generated by the one or more transmitting acoustic transducers, wherein signal components corresponding to said flexural mode are isolated from signal components of other modes by using a gating technique applied to the received signals; and the processing unit is operable to analyze a group velocity and a phase velocity of said flexural mode in a range wherein a group velocity of said flexural mode is approximately constant, said processing unit being operable to compute from a change in the said phase velocity a change in a true thickness of said solid structure. (a) transmitting an acoustic Lamb wave signal from at least one of said acoustic transducers into a wall of said structure and receiving the acoustic signal at a plurality of said transducers after propagating a distance within the solid; (b) collecting signals from said plurality of transducers in processing unit; (c) repeating steps (a) and (b), transmitting and receiving from said plurality of transducers; and (d) analysing the received signals generated in steps (a) - (c); (e) analysing a group velocity and a phase velocity of said Lamb wave propagation mode of said signal with the processing unit; and (f) computing a change in a true thickness of said solid structure; characterized in the following steps, the signal received by the receiving acoustic transducer and analyzed in the processing unit is a fundamental flexural mode generated by the one or more transmitting acoustic transducers, wherein signal components corresponding to said flexural mode are isolated from signal components of other modes by using a gating technique applied to the received signals; and the processing unit is operable to analyze a group velocity and a phase velocity of said flexural mode in a range wherein a group velocity of said flexural mode is approximately constant, said processing unit being operable to compute from a change in the said phase velocity a change in a true thickness of said solid structure.