Image time series for functional studies frequently vary relatively little from one time point to the next. We describe here a method of LOSS-LESS compression that calculates the change in signal intensity as a char (one-byte) as opposed to a short or long integer (usually two or four bytes), thus resulting in a compressed file size just over half the size of the original. The calculations and format are exceedingly simple; the compression time is comparable to the time required for a file copy, and is thus transparent to most users. While designed for FUNCTIONAL MRI, the approach works equally well on any integer-based scanning modality, such as PET, CT or DSA. Our purpose in this communication is to advocate it as a standard, to promote free interchange of scan data.