Phase-resolved optical spectroscopy of the recently identified Intermediate Polar (IP) RX J1548-4528 (Haberl et al. 2002) reveals radial velocity variability of emission lines at the 693 s X-ray and photometric period, confirming the identification of the white dwarf (WD) spin period as well as at a 9.87 hr period, which we identify as the orbital period. RX J1548-4528 is then the fourth longest period IP known so far. The optical spectrum is peculiar with broad absorptions around Balmer emission lines resembling those observed in V 709 Cas (Bonnet-Bidaud et al. 2001). Their origin, the WD or disc, remains unclear. An evolved G9-K3 secondary star is detected in the red spectra.