We propose a cross-correlation technique for measuring ultrashort soft x-ray pulse shapes using the rapid increase in Kr+ ion density caused by optical field-induced ionization, which operates as an ultrafast x-ray absorption switch. Using this technique, we measured the shape of a soft x-ray pulse near 15.6 nm emitted from W plasma produced by a 100 fs laser pulse, and found the duration to be about 4 ps assuming a Gaussian pulse. This result was in good agreement with the duration measured with an x-ray streak camera thus confirming the feasibility of our technique. The temporal resolution of this technique has the potential to overcome the limitation of the ionizing pulse duration. © 2001 American Institute of Physics.