This thesis investigates how John Keats explores the diverse and continually shifting relationships between pleasure and pain. It considers how far Keats engages with, advances, and departs from a medical understanding of affective experience by thinking about how poetry becomes the means by which Keats tests, explores, and experiments with the idea that pleasure and pain are intrinsically linked. This study hones in on Keats's most productive years of poetic composition, between 1818 and 1820, as a period in which Keats intensely experimented with poetic form and genre even as he experienced a painful decline in health. Through a new formalist and reader response approach, this thesis shows how the nuances of poetic language and the subtle manipulations of poetic form are the spaces through which Keats navigates the dynamic nature of 'aching Pleasure' ('Ode on Melancholy', 23). The opening chapter focusses on weeping in Isabella, analysing the liminal status of tears as psycho-physiological manifestations that encompass both the pleasures and pains of the mind and body. The second chapter continues to look at selfhood and grief in Hyperion. A Fragment, focussing on the figure of Saturn to discuss how Keats tests the notion that identity can be shaped and perfected by pain and loss. Chapter three moves away from individual suffering to investigate how The Fall of Hyperion and The Eve of St Agnes situate the reader on the outside of spectacles of pleasure and pain, highlighting the unsettling pleasure experienced during the act of observing another's encounter with anguish or delight. In chapter four, the uncertain and 'bitter-sweet' (Lamia, I, 59) 'truth' status of pleasure is explored in Lamia; a poem that teasingly provokes analytical choice-making at the same time as it refuses to take sides. The final chapter looks at Keats's 1819 spring odes to show how numbness is not an experience that is analogous to the annihilation of all feeling, but a sensation that incorporates the elusive tinglings of pleasurable pain. Keats's poetry never defines a single theory of how pain and pleasure intermingle. Instead, it relishes in exploring painful pleasure as an uncertain affective state, demonstrating the inexhaustible diversity of human experience. This thesis shows how Keats's profoundest insights and most intellectually challenging lines of poetry occur at those moments when the young poet attempts to 'unperplex bliss from its neighbour pain' (Lamia, I, 192).