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John Acocks' ideas on the recent and future changes of vegetation in South Africa occupy an important conceptual position in the first and subsequent editions of Veld Types of South Africa. Arguably the most prominent idea embodied in his discussion of vegetation changes is the hypothesis that various Karoo veld types were expanding eastwards and northwards into areas that would otherwise be occupied by grassland, and that this process was a direct result of mismanagement of the semi-arid rangelands. The invasion of Karoo vegetation amounts to a systematic consideration in South Africa of what would ultimately become known as desertification. This paper examines Acocks' interpretation of the vegetation changes associated with expanding Karoo and desert veld types and his evidence that this was indeed occurring. It places the apparent degradation into an appropriate historical context in relation to the evolving views of other South African and international scientists. The paper shows how Acocks' understanding of vegetation dynamics influenced agricultural policy, both immediately following publication of Veld Types and subsequently. Finally, a critique of the spreading Karoo hypothesis is developed in relation to current theory on desertification and its implications for management of the semi-arid rangelands of South Africa today.