Amphiphilic diblock copolymers, poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA-PAA), with narrow molecular weight distribution (PDI <= 1.07) were prepared by atom transfer radical polymerization (ATRP) of n-butyl acrylate and tert-butyl acrylate (tBA), followed by selective acidolysis of the PtBA block. These polymers possess a soft PnBA hydrophobic block with a constant chain length of 90-100 monomer units and pH- and ionic strength-sensitive hydrophilic PAA block with DPPAA = 33-300 AA monomer units. They were expected to form stimuli-responsive micelles. The block copolymers with DPPAA ! 100 are directly soluble in water at pH > 4.7. Pyrene steady-state fluorescence spectroscopy and fluorescence correlation spectroscopy (FCS) studies indicate the existence of a very low critical micelle concentration (cmc similar to 10(-8) mol/L). The number-average hydrodynamic radii of the micelles, as determined by FCS, range from 28 to 55 urn, depending on the PAA block length. FCS data indicate that micellar sizes significantly decrease upon dilution for salt-free systems. This is attributed to a dynamic, but kinetically controlled, behavior of these self-assembled nanostructures. In saline solutions the micellar sizes remain constant above the "apparent" cmc (cmc*), which is attributed to slower dynamics of unimer exchange between micelles.