Electrochemical hydrogen permeation tests were utilized to investigate the diffusion and trapping of hydrogen in API X70 pipeline steel. Thermomechanical processing was used in order to produce 4 samples with different microstructural features. Detailed characterization was performed to measure the size and fraction of microstructural constituents of the samples and their effects on hydrogen diffusion were studied. After thermomechanical treatment the samples contained pearlite, polygonal ferrite, acicular ferrite, and martensite/austenite islands, the size and amount of which depends on the processing parameters. Hydrogen trapping density and hydrogen diffusion coefficient were measured for each sample and it is showed that depending on the properties of the microstructural constituents the diffusion coefficient can vary in the range of 0.33 x 10(-6) cm(2)/s to 1.61 x 10(-6) cm(2)/s.