This paper forms part of an exploration of assessment on one part-time higher education (HE) course: an in-service, professional qualification for teachers and trainers in the learning and skills sector which is delivered on a franchise basis across a network of further education colleges in the north of England. This paper proposes that the validity and reliability of portfolio-based assessment, a key component of many HE programmes in addition to the course being researched here, is contestable. Analysis of the processes of compiling portfolios for assessment, through the conceptual framework of the New Literacy Studies, suggests that the ways in which portfolios are assessed and the ways in which the crucial requisites of validity and reliability are assigned to them, mask complexities and contradictions in their creation by the student. This paper argues for a new, critical analysis of portfolio production and raises a number of questions about the validity, reliability and authenticity of the assessment process that the portfolios reify.