Twenty samples from soils developed over the Northampton Sand ironstone formation were collected from, in and around the town of Wellingborough, Northamptonshire, UK. The total arsenic (As) content ranged from ca. 20-100 mg kg^{- 1}and the bioaccessible As content, as measured by a physiologically based in vitro extraction test, ranged from 1 to 6 mg kg^{- 1}. A chemometric algorithm for mixture resolution, when applied to total element and total organic carbon concentration of the soils, was able to identify chemically distinct soil constituents and their associated As content. Multiple linear regression (MLR) modelling, using the As content of the intrinsic soil constituents and their first order interactions as independent variables, was able to predict the bioaccessible As content of the soils (R² = 0.85) with an uncertainty of 1.33 mg kg^{- 1}. Although the MLR model showed that the interactions between the soil constituents were the key factors controlling the bioaccessible fraction in each soil most of the total As was found to be bound to an Fe oxide soil constituent. The model predictions shown are currently only valid for the geological and soil chemical setting investigated here, extrapolation to other geological settings would require additional investigations.