Dataset associated with the publication "Onboard carbon capture for circular marine fuels" by Margarita A. Charalambous, Valentin Negri, Valentin Kamm, and Gonzalo Guillén-Gosálbez, published in the journal ACS Sustainable Chemistry and Engineering. Link to publication The dataset includes all the codes, environmental databases, simulations, figures, and economic data. The codes and the excel data are required to reproduce the plots embedded in the main manuscript. The dataset includes 3 Excel files. The content of each dataset is here elucidated: Data.xlsx: Inventory datasets used for the economic assessment. Includes the costs per tonne kilometer (tkm) for methanol and natural gas ships, and also the final data, including the sensitivity analysis used for plotting. Circular-marine-fuels-2030.xlsx: life cycle inventories used for 2030. Includes the production of CO2, H2, methanol, natural gas, and the transport inventories for the three ships (methanol, natural gas, BAU). This is an excel file database that can be directly used in Brightway, using Activity browser. The database was created using data from premise compatible with Ecoinvent v3.10. Circular-marine-fuels-2050.xlsx: life cycle inventories used for 2050. Includes the production of CO2, H2, methanol, natural gas, and the transport inventories for the three ships (methanol, natural gas, BAU). This is an excel file database that can be directly used in Brightway, using Activity browser. The database was created using data from premise compatible with Ecoinvent v3.10. The dataset includes 2 python jupyter notebooks. The content of each dataset is here elucidated: env_calc.ipynb: Python code that shows how the calculation of the life cycle impact assessment is performed and how the results are plotted.Plotting.ipynb: Python code that shows how to plot the economic results. The dataset includes 4 process simulations. The content of each simulationis here elucidated: MeOH_Production.hsc: Aspen HYSYS file that includes the process simulation of methanol production, using CO2 from ships and DAC. Capture_onboard_methanol.hsc: Aspen HYSYS file that includes the process simulation of capture onboard methanol ship. Capture_onboard_NG.hsc: Aspen HYSYS file that includes the process simulation of capture onboard natural gas ship. LNG_Production.apw: Aspen PLUS file that includes the process simulation for natural gas production.