Macrovoids are defects that are frequently encountered in phase inversion as well as in other solution-cast membranes. Macrovoid formation continues to hold many surprises despite several decades of pertinent studies and frequent claims for definitive answers. We have discovered conditions leading to macrovoid growth parallel to a transparent membrane skin (horizontal macrovoids, HMVs); the benefits from the macrovoid-study point of view are slow growth, easy observation and possibility of designing modes of interference that facilitate the understanding of the parameters that affect the initiation and growth (including orientation) of macrovoids. Our study emphasizes the case of dopes consisting of atactic PMMA dissolved in acetone and solidified in a hexane non-solvent bath but dopes of other polymers also exhibit the same features. While a host of parameters affect the occurrence of HMVs we find that they are ultimately a possible response to the mechanical force field that evolves during progressing solidification of the concentrated polymeric solution; as a result HMVs can occur, under appropriate conditions, regardless if there is phase separation (which leads ultimately to a macroporous bulk membrane microstructure) or not. (C) 2012 Elsevier B.V. All rights reserved.