Abstract: Nuclear speckles organize into 20-50 irregularly shaped structures on the sub-micron-scale and are enriched in serine/arginine-rich splicing factors (SRSFs) involved in mRNA splicing. Speckle-associated RNA binding proteins (RBPs) have multi-domain architectures comprising one or two folded RNA recognition motifs and intrinsically disordered regions. The molecular architectures of SRSFs map onto di- or tetra-block copolymers, driving microphase separation above a threshold concentration. Accordingly, we observe the formation of size-limited, ordered assemblies that result from a balance of attractive and repulsive interactions among the SRSF domains. Structural investigations using freeze-fracture deep-etch electron microscopy show assemblies of 30-40 nm in diameter with distinct morphologies. We also observe that microphases can associate to form sub-micron scale clusters by short-range attractive and long-range repulsive interactions that are disrupted by the presence of a co-ion. We also find that the speckle-enriched long non-coding RNA MALAT1 and the C-terminal domain of the RNA polymerase II promote microphase separation of SRSF1 by lowering the threshold concentration. This suggests a model for a molecular mechanism of interplay among the essential components involved in RNA processing.