The highly branched, three-dimensional geometry of dendritic macromolecules [1] makes these new molecular architec-tures ideal container molecules. [2] It has been suggested that these molecules could be used in a number of applications including those related to the controlled release of pharmaceuticals. [3, 4] Several host ± guest systems have already been developed, for example, dendritic hosts with unimolecular (inverted) micellar structures, [5] the ªdendritic boxº, [6] crown ether dendrimers, [7] and cyclophane dendrimers. [8] A restricted number of guests, such as rose bengal, can be encapsulated in the ªdendritic boxº, (a fifth generation poly(propylene imine) dendrimer modified with a dense shell of amino acids) [6] and released by simple chemical modification of the shell. [6c] Dynamic hosts in organic media [9a] or supercritical CO 2 [9b] are based on hydrophobically modified poly(propylene imine) dendrimers and have proved to be efficient extractants of aqueous solutes. Recently, more attention has been focussed on water-soluble dendritic systems, [10] but their host ± guest properties have not been addressed so far. Herein we present poly(propylene imine) dendrimers modified with 3,4,5-tris(tetraethyleneoxy)benzoyl units, which have a basic interior of tertiary amines and a hydrophilic periphery (Scheme 1). Titrations and small angle X-ray scattering Scheme 1. Top: Synthesis of oligoethyleneoxy-functionalized poly(propylene imine) dendrimers; n 4: 1; n 16: 2; n 32: 3, and n 64: 4. Bottom: Schematic structure of host 2.