It is generally accepted that priming causes some degree of loss of shelf life. Germains priming methods do not cause a reduction in viability after storage for 18 months at 20°C and 55% RH, which is considered to represent 1-year carryover of seed held according to conventional good practice. However, the majority of seedlots suffer some degree of loss in speed expressed as time to 50% germination. This loss is paralleled by non-primed seed, such that the difference in speed between primed and check seed (‘advancement’) is preserved. Introduction: It is accepted knowledge for many species that while priming results in both faster and more uniform germination it causes some degree of loss of shelf life. While there are reports of longevity increased by priming of several species (e.g. Wood & Hay, 2010) the effect on high vigour seed, such as commercial sugar beet seedlots, is generally to reduce shelf life (Varier et al. 2010). Development of commercial priming protocols must take account of the shelf life requirement for each individual seedlot and finished product form. For sugar beet, the objective for Germains has been to ensure seed remains suitable for planting if carried over to the following season. In practice, that equates to a period that can extend to 18 months since the commercial priming season typically starts in November while sowing is generally completed by April. Shelf life is often determined simply as loss of viability, but to varying degrees there is also a progressive loss of germination speed with increasing period of storage. This is of particular significance for primed seed where speed gained through priming is potentially at risk Storage characteristics of sugar beet seed have not been characterised in detail, but appear to follow common rules that can be applied to all species. On that basis, shelf life is predicted to reduce by half for (1) every 1% increase in moisture and (2) every 10°F (5.6°C) increase in temperature (Bewley & Black, 1994). Hence, to fully characterise changes in sugar beet seed during storage, it is essential to control both temperature and moisture content. We have stored seed under a variety of controlled conditions but experience suggests 20°C at 55% relative humidity (RH) is representative of conventional good storage practice. * Corresponding author, sharper@germains.com Materials and Methods: Samples of pelleted seed were held at 20°C in sealed plastic containers over saturated magnesium nitrate solution to control RH at approximately 55%. Individual samples of 300 seeds were held in plastic trays, as a single layer deep, to ensure every seed was maintained in water activity equilibrium with the headspace.