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[en] To provide two jitter-free soft x-ray pulses for femtosecond x-ray pump and probe experiments a split and delay unit (autocorrelator) has been constructed for the VUV-FEL in Hamburg (FLASH). Here we report experiments applying this autocorrelator to examine the average temporal properties of FEL pulses delivered from FLASH at 24 nm (51.8 eV). In a linear autocorrelation experiment the spatio-temporal coherence properties are measured for both the first and the third harmonic of the FEL pulses. Furthermore, we report on the first evaluation of the pulse length from the time-resolved observation of doubly charged helium ions produced by direct two-photon double ionization at 24 nm. In summary the determination of the longitudinal pulse parameter of FLASH at 24 nm to 6 fs and 29±5 fs for the coherence time and the pulse length (FWHM) respectively proofs the autocorrelator as a valuable tool for time resolved two pulse X-ray experiments.