Journey into Mystery (vol. 1) was a comic that began publication in 1952 by Atlas, and although it wasn't until the eighty-third issue that Thor even appeared in comics, let alone the title, it is most often linked to his character. This is why after releasing the one hundred and twenty-fifth issue of the series in 1966 that the series changed its name to become Thor (vol. 1). Journey into Mystery (vol 2.) was started by Marvel in 1972 and ran for 19 issues. For more than three hundred issues the title remained as Thor, but in 1996 after the five hundred and second issue of the series, they decided to change the name back to Journey into Mystery (vol. 3) and continued Thor (vol. 1) numbering until 1998 with the five hundred and twenty-first issue. Although these later Journey into Mystery issues (503-521) spin out of Thor vol.1, they are not counted in the collective Thor series.
Note: Although the series spins out of Thor vol.1, it is not counted in the collective Thor series.