A carpetlike dense-layer formation between a hydrophobic layer and a polyelectrolyte brush layer has been found in the monolayers of an ionic amphiphilic diblock copolymer, poly(1,1-diethylsilacyclobutane)(m)-block-poly(methacrylic acid)(n), on a water surface by an X-ray reflectivity technique. By detailed analysis, we have found that the hydrophilic layer under the water is not a simple layer but is divided into two layers, that is, a carpetlike dense methacrylic acid (XLA-A) layer near the hydrophobic layer and a polyelectrolyte brush layer. We have also confirmed that a well-established polyelectrolyte brush is formed only for the m:n = 43:81 polymer monolayer: For m:n = 40:10 and m:n = 45:60 polymer monolayers, only a dense MAA layer is formed. This dense-layer formation should be the origin of the interesting hydrophobic-layer thickness variation previously reported; The hydrophobic-layer thickness takes a minimum as a function of the hydrophilic chain length at any surface pressure studied. An over-view of the data for three samples with different chain lengths (m:n = 40:10, 45:60, or 43:81) has shown that the thickness of this dense layer is 10-20 Angstrom and is independent of the surface pressure and polymerization degree of poly(methacrylic acid) (PMAA) in the range studied. This dense-layer formation is explained by the reasonable speculation that contact with PMAA is thermodynamically more stable than direct contact with water for the diethylsilacyclobutane (Et2SB) layer on water. In this sense, the dense layer acts like a carpet for the hydrophobic Et2SB layer, and a 10-20-Angstrom thickness could be a critical value for the carpet. (C) 2003 Wiley Periodicals, Inc.