This dataset consists of a variety of hydrographic, biogeochemical and meteorological data. Hydrographic profiles, towed and underway measurements and point sources provided information on free-fall turbulence data, current velocities and acoustic backscatter, water column structure including temperature and salinity, the underwater light field, fluorescence and dissolved oxygen. A comprehensive biogeochemical water sampling programme provided details on nutrients, primary productivity, dissolved organic matter and phytoplankton pigments. Biological samples such as zooplankton were obtained from the water column using nets, and from the sea-bed using grabs. Bathymetry and meteorological parameters were measured across the study area. A dye release experiment was also carried out. Data collection was undertaken in the Celtic Sea. The data were collected during the period 02 - 27 July 2008 during RRS James Cook cruise JC025. Measurements were taken using a variety of instrumentation, including conductivity-temperature-depth (CTD) profilers with attached auxiliary sensors, bathymetric echosounders, water bottle samplers, nets, acoustic Doppler current profilers (ADCPs), remote access water samplers, towed undulators, free-fall turbulence profilers, temperature loggers, fluorometers, grabs and ship flow-through and meteorological packages. The data have been collected as part of the United Kingdom (UK) Natural Environment Research Council (NERC) Oceans 2025 programme (Work Package 3.2) to provide information on vertical mixing processes at the thermocline. This will help improve modelling of these processes and is an expansion of work carried out during a previous National Oceanography Centre Liverpool (NOCL) project ‘Physical-Biological Control of New Production within the Seasonal Thermocline’. The cruise was undertaken jointly by NOCL, the Scottish Association for Marine Sciences (SAMS), the University of Aberdeen, the University of Strathclyde, Napier University and the Joint Nature Conservation Committee (JNCC). The Principal Scientist during the research cruise was Professor Jonathan Sharples of NOCL, who is also the Principal Investigator of Work Package 3.2. CTD data, towed undulator data, temperature logger data, nutrient data, ADCP data, dye tracking data, zooplankton data, primary productivity data and ship underway monitoring system data from this cruise are held at the British Oceanographic Data Centre. Other data have not yet been supplied.