Abstract.Let T be a completely regular Hausdorff space, Cb(T) the space of bounded continuous real-valued functions on T, M(T) the Banach space dual of Cb(T). Let % denote the family of subsets of Cb(T) which are uniformly bounded and relatively compact for the topology 9^ of pointwise convergence. A well-known theorem of Grothendieck [28] asserts that if F is a compact Hausdorff space, and H is a uniformly bounded, pointwise compact set of real-valued continuous functions on F, then H is compact in the weak topology of the Banach space CÍT). Thus, in this case, the weak and pointwise topologies agree on//. This article is concerned with extensions of Grothendieck's Theorem to the setting: F is a completely regular Hausdorff space, and CbiT) is the Banach space of bounded continuous real-valued functions on T, with the supremum norm. Ptak [51] and Tomasek [64] have shown that Grothendieck's Theorem as stated above holds if and only if T is pseudocompact. However, one can pursue the matter as follows:Main Problem. For a given completely regular T, what is the largest linear subspace Z of MÎT), the Banach space dual of CbiT), such that the pointwise topology and the weak topology