The morphology of conjugated polymers strongly influences their optical and electronic properties and affects their performance in polymer devices. Using optical spectroscopy and atomic force microscopy, we investigate the fluorescence properties and the aggregation state of DNA-functionalized poly(phenylene-vinylene). We show that polymer aggregation can be controlled in solution through ion and DNA interactions; aggregation is induced in the presence of divalent cations and can be reversed by adding sequence specific DNA. These interactions provide ways to tune polymer aggregation on the timescale of minutes and allows tuning of the polymer's optical properties.