Systematic study of data sharing by citizen scientists will make a significant contribution to science because of the growing importance of aggregated data in data‐intensive science. This article expands on the data sharing component of a paper presented at the 2013 ASIST conference. A three‐phase project is reported. Conducted between 2011 and 2013 within an environmental voluntary group, the Australian Plants Society Victoria (APSV), the interviews of the first phase are the major data source. Because the project revealed the importance of data sharing with professional scientists, their views are included in the literature review where four themes are explored: lack of shared disciplinary culture, trust, responsibility and controlled access to data, and describing data to enable reuse. The findings, presented under these themes, revealed that, within APSV, sharing among members is mostly generous and uninhibited. Beyond APSV, when online repositories were involved, barriers came very strongly into play. Trust was weaker and barriers also included issues of data quality, data description, and ownership and control. The conclusion is that further investigation of these barriers, including the attitudes of professional scientists to using data contributed by citizen scientists, would indicate how more extensive and useful data sharing could be achieved.