The dataset contains physical, biogeochemical and biological data, including measurements of water temperature, salinity, fluorescence, dissolved gases and current velocities; plankton samples from nets and plankton recorders; water samples for analysis of nutrients, phytoplankton, radioactivity and biogeochemical parameters; benthic cores; meteorological time series (pressure, temperature, humidity, wind velocities); atmospheric samples and ocean-atmosphere fluxes; and results from incubation experiments. The data were collected north of the Crozet Plateau in the Southern Ocean/Southwest Indian Ocean on RRS Discovery cruises D285 (3rd November - 10th December 2004) and D286 (13th December 2004 - 21st January 2005). Much of the data collection focussed on a series of Major Stations (called M1 to M10), with measurements being collected at these stations every two or three days. Conductivity-temperature-depth (CTD) casts were undertaken at each station, providing both hydrographic data and water samples from a range of depths. Other work at each Major Station included zooplankton nets, Longhurst-Hardy Plankton Recorder (LHPR) tows, sediment coring and Argo float deployment. In between Major Stations some additional CTD casts were undertaken. The SeaSoar oceanographic undulator provided further hydrographic data, while hull-mounted acoustic Doppler current profilers (ADCPs) provided current velocity data across the survey area. In addition, continuous underway measurements of hydrographic and meteorological parameters and surface water samples were collected along the cruise track. Five moorings were deployed, one of which was recovered at the end of D286. The other four, including sediment traps, current profilers and CTDs were deployed for one year. CROZEX (CROZet circulation, iron fertilization and Export production experiment) is a complex, multidisciplinary project to examine, from surface to sediment, the structure, causes and consequences of a naturally occurring annual phytoplankton bloom that forms. This collaborative project involved researchers in Ireland and the UK, and was administered by the National Oceanography Centre (NOC), Southampton. Data are managed by the British Oceanographic Data Centre. Much of the CROZEX data processing is ongoing and a number of datasets have yet to be submitted to BODC. The data described here are those presently held by BODC, with the exception of the Argo floats (these data are not expected by BODC and should be accessible via the Argo website) and the four year-long mooring deployments (data from these will be submitted to BODC in the future).