From a specifications perspective, the probes are identical. Both are 500 MHz, 10X passive probes. The P6139B can be used on oscilloscopes that shipped with the P6139A. From a mechanical perspective, there are some differences: - Sharper tip - Ground spring attaches directly to the tip without the special sleeve used in the P6139A - Alligator Ground lead attaches directly above the plastic sleeve and does not use the ground lead with the "donut" - Sleek design - Compliant to March 2011 safety updates (UL approved) Note: P6139A (3.5mm probe tip) and P6139B (3.8mm probe tip) have different diameter probe tips and cannot share probe accessories. The TPP0100, TPP0101, TPP0200, TPP0201, TPP0500, TPP1000, and P6139B passive probes all share a common probe tip diameter, so the probe accessories work across all of the probes.